CHAPTER-1-Introduction-and-Background_1995_Studies-in-Inorganic-Chemistry.pdf

CHAPTER 1

INTRODUCTION AND BACKGROUND

Phosphorus, symbol P, atomic number 15, atomic weight 30.97, belongs to Group V of the periodic table of the elements. The elements of this group, namely nitrogen, phosphorus, arsenic, antimony and bismuth, are sometimes known as pnicogens or pnictides.

The chemistries of phosphorus and nitrogen are by far the largest and the most important amongst those of the pnictide elements. Phosphorus and nitrogen are by far the most abundant pnictides and, unlike arsenic antimony and bismuth, they play an essential role in life processes and the environment.

The chemistry of phosphorus generally resembles that of arsenic much more closely than that of nitrogen, and the latter stands somewhat apart from the other elements of the group. Phosphorus and arsenic frequently form similar compounds whereas phosphorus and nitrogen seldom do.

Phosphorus chemistry is dominated by oxyphosphorus compounds all of which contain phosphorus-oxygen l inkages. Most of these are usually known as phosphates. Almost all natural ly occurring phosphorus compounds contain phosphorus-oxygen linkages, and those of biochemical importance are organic phosphate esters which contain phosphorus-oxygen-carbon l inkages. Organophosphorus compounds (carbophosphorus compounds) .which are based on phosphorus-carbon linkages, constitute the second most important group and those containing phosphorus-nitrogen linkages (azaphosphorus compounds) are probably the third. Metallophosphorus compounds which contain metal-phosphorus linkages also constitute a large and rapidly growing group. Compounds belonging to one or more of these groups are exceedingly numerous and greatly outnumber all remaining compounds formed by the element.

The mineral Apatite is the most abundant and widespread phosphorus compound on earth and phosphoric acid is the most important industr ial commodity based on phosphorus. However, the organic phosphate ester known as deoxyribonucleic acid (DNA), is present in all life forms and lies at the very heart of biochemistry and genetics. It is the most-studied phosphorus compound and is probably the most crucial phosphorus compound as far as the survival and development of the human race is concerned.

1.1 HISTORICAL

The discovery of phosphorus is generally attributed to Hennig Brand of Hamburg, who in 1669 obtained it by distil l ing urine. The substance he obtained glowed in the dark and burst into flame when exposed to a i r . It was subsequently named 'phosphorus ' , meaning l ight-bearing. Arabian alchemists may have discovered the element much earl ier , but the term 'phosphorus' was often applied to various l ight-bearing materials which were devoid of the element. Examples were 'Bologna Phosphorus'- a luminescent barium sulphide, and 'Baldwin's Phosphorus' - a luminescent calcium ni t ra te . Many modern phosphorescent materials do not contain the element. Microcosmic salt (sodium ammonium hydrogen phosphate) .known since ancient times, was probably one of the earliest phosphorus compounds to be prepared.

2 1.1

The discovery of elemental phosphorus was soon followed by the characterisation of its combustion> product, phosphorus pentoxide, and in 1694 Boyle prepared phosphoric acid by dissolving the latter in water. About 1770 phosphorus was recognised by K.W. Scheele as an essential ingredient of animal bones and teeth, when he prepared the element from bone ash, carbon and sand. By 1779 the first phosphorus-containing mineral, Pyromorphite, had been identified by Gahn.

During the first half of the nineteenth century significant advances were made in the science of plant nutrition, notably by Liebig and Lawes, and the value of phosphates as fertilizers was soon realised. In 1842 Lawes and Murray took out British patents for the manufacture of fertilizers from sulphuric acid and bones. By this time several simple inorganic phosphorus compounds had been characterised. Among these were phosphine in 1783 by Gengembre, and phosphorus trichloride in 1808 by Gay Lussac and Thenard. The historical classification of orthophosphates, pyrophosphates and metaphosphates was introduced by Graham in 1833.

The first phosphorus-containing striking matches were invented by Sauria in 1831, and the use of the element in match compositions was introduced in England and France about 1838. By the end of the century matches and fertilizers had become firmly established commercial uses for phosphorus compounds.

Although urine remained the only source of the element for nearly 100 years after its discovery, it had been replaced by bones by the end of the eighteenth century. Supplies of the latter soon proved to be inadequate, but fortunately substantial phosphate mineral deposits were quickly found.This enabled serious commercial production of phosphorus compounds from these ores to commence in Europe about 1850, when 'wet process' phosphoric acid became avai lable. In 1888 a major development took place when Readman invented the electric furnace method for the continuous production of the element directly from phosphate ores. In 1890 the first industr ial electric furnace came into use at Oldbury in England, and this was followed later by similar production at Niagra Falls, USA.

The first organic phosphorus compound to be identified was probably lethicin, isolated from brain fat in 1811 by Vauquelin, and characterised as a phosphorus-containing lipid by Gobley, in 1850. In 1868 another natural ly occurring organic phosphorus compound, 'nucle in ' , was isolated by Miescher from pus cells obtained from surgical bandages. This represented an important step in the association of phosphorus compounds with living t issues.

The earliest laboratory synthesis of an organic phosphorus compound was reported by Lassaigne, who in 1820 obtained crude alkyl phosphates by reacting alcohols with phosphoric acid. This was followed by the synthesis of phosphine derivatives by Thenard in 1845, and by the end of the century quite a number of compounds containing both phosphorus and carbon had been produced. The most notable pioneers in this field were Michaelis (1847-1916) and Arbusov (1877-1968) who are now J regarded by many as the founders of organophosphorus chemistry.

The most remarkable advances in twentieth century biology and biochemistry are connected with compounds containing both phosphorus and carbon. The universal energy transfer compound, adenosine triphosphate (ATP), first discovered by Fiske and Subarrow in muscle in 1929, was synthesised some 20 years later by Todd and co-workers.

1.1 3

Following the elucidation of the glycolysis process by Embden and Mayerhof in 1932, and the glucose oxidation process by Krebs in 1937, the concept of high-energy phosphate bonds was introduced by Lipmann in 1941. By this time the intimate involvement of phosphorus compounds in numerous biochemical reactions had been firmly demonstrated.

Schrader and Saunders on the eve of World War II independently discovered the highly toxic properties of certain phosphate esters. This led, in the ensuing decades, to their intensive development tooth as insecticides and nerve gases.

By 1:940 it had been clearly established that the highly polymerised phosphate esters known as nucleic acids and recognised as the essential components of 'nucle in ' , were the normal constituents of all life cells. Furthermore, it was realised that these compounds were the essential constituents of the chromosomes, long recognised for their function in hereditary processes.

The elucidation of the molecular struture of the nucleic acids (DNA & RNA) by Crick and Watson in 1953, probably represents the mjost profound achievement of twentieth century biology. Another great achievement was the discovery in 1973 of DNA 'cloning' techniques, which has opened up the enormous possibilities of genetic engineering.

It is now accepted that phosphorus compounds play a vital role in living processes and are essential, not only for hereditary processes, but for the growth, development and maintenance of all plants and animals. They are present in soil, bones and teeth, and in blood and all cellular organisms. Energy transfer processes such as photosynthesis, metabolism, nerve function and muscle action all involve phosphorus compounds. Reversible phosphorylation is the most universal mechanism employed in nature for regulating the action of enzymes and other proteins involved in biochemical processes.

Phosphorus compounds are essential for nitrogen fixation and the operation of the natural nitrogen cycle, which enables the latter element to be assimilated by plants .

The expansion of organophosphorus chemistry has been considerable since stable multiply-bonded phosphorus-carbon compounds were synthesised for the first time by Gier and by Burg and Mahler in 1961. Expansion has been part icularly great over the past decade and it is mow evident that there exists a huge chemistry based on phosphorus-carbon linkages and involving many types of bond configuration about the P atom. Although . much of this remains unexplored and as yet without commercial application, progress continues at an explosive rate and the future importance of this branch of phosphorus chemistry should not be underestimated.

The past decade has also witnessed a very rapid growth of metallophosphorus chemistry and the realisation that phosphorus is capable of forming multiple bonds to many elements other than carbon. In addition,an ever widening involvement of phosphorus compounds in biochemical processes continues to be uncovered.

The 20 t n century expansion of all phosphorus chemistry has been enormous and it has been paralleled by a great increase in both the diversity and volume of application of manufactured phosphorus compounds. Among these applications,however, a handful of relatively simple inorganic phosphorus compounds remain the most prominent.

In the present world the leading industr ial phosphorus chemical is phosphoric acid, and on a tonnage basis the inorganic oxyphosphorus compounds (phosphates) remain the most important, with

4 1.2

f e r t i l i z e r s cons t i t u t i ng the l a r g e s t s ing le a p p l i c a t i o n (85-90%) .Synthe t ic de t e rgen t s come second and an imal foodstuffs t h i r d . The o rgan i c compounds, commercial ly impor tan t s ince about 1940, have numerous a p p l i c a t i o n s , p a r t i c u l a r l y in p l a s t i c s and i n s e c t i c i d e s , but a t p r e sen t u t i l i s e l i t t l e more t h a n about 5 % of the phosphorus manufac tu red .

Presen t f ie lds of use of P compounds inc lude :

Animal foodstuffs Auto rad iography Biochemical r e s e a r c h Bui ld ing m a t e r i a l s C a t a l y s t s Chemical s y n t h e s i s Chromatography Criminology Dental m a t e r i a l s Dess icants Detergents

E lec t rop la t ing E lec t r i ca l m a t e r i a l s F e r t i l i z e r s Flame r e t a r d a n t s Food a d d i t i v e s Genetic eng inee r ing Glass technology Luminescent phosphors Matches Medicines Metal t r ea tment

Nerve g a s e s Oil a d d i t i v e s Pes t i c ides Pigments P l a s t i c s Refrac tor ies Smoke gene ra t ion Solvent ex t r ac t ion S u r f a c t a n t s Toothpas te Water t r ea tmen t

1.2 DISTRIBUTION AND ENVIRONMENT

Phosphorus is not found free in n a t u r e and almost a l w a y s occurs in the ful ly oxid ised s t a t e a s p h o s p h a t e . The element i s widely d i s t r i b u t e d in t h i s form in so i l s , rocks , in the oceans , in a l l l i v i n g ce l l s , in most foods and in many man-made m a t e r i a l s where n i t rogen is a lso u s u a l l y p r e s e n t . Unlike the l a t t e r , however, phosphorus i s g e n e r a l l y absen t from the a tmosphere .

The n a t u r a l a b u n d a n c e s of the pn i c t i de elements l i e in the same order a s t he i r atomic weights :

N > P > As > Sb > Bi The i n d u s t r i a l p roduct ion of these elements and t h e i r compounds a lso l ies in approx ima te ly the same order (Tables 1.1 & 1.2) .

Nitrogen is the most a b u n d a n t pn ic t ide element in the so la r system, in the s ea , in the a i r , in the soil and in l i v i n g o r g a n i s m s . I t does , however, a p p e a r to t a k e second p lace to phosphorus in the e a r t h ' s c r u s t a l r o c k s . The r ema in ing pn i c t i de e lements , a r s e n i c , ant imony and bismuth , a r e a l l p resen t in c o n s i d e r a b l y smal ler q u a n t i t i e s t han e i the r n i t rogen or phosphorus in a l l these media (Tables 1.1 & 1.3) .

The pn ic t ide elements a r e l ess p len t i fu l t h a n carbon almost everywhere except in the a tmosphere . In the l a t t e r n i t rogen is more plent i fu l t h a n ca rbon , but a l l the h e a v i e r pn i c t i de elements a r e v i r t u a l l y absen t . Although the q u a n t i t i e s of p n i c t i d e s p r e sen t in the oceans a r e impress ive when cons idered in terms of t o n s / c u b i c mile of s eawa te r (N = 2400, P = 330, As = 14, Sb = 2, Bi = 0.1) these concen t ra t ions a r e too low to r ende r t he i r ex t r ac t i on commercial ly f e a s i b l e . Phosphorus has been detected in i n t e r s t e l l a r space but more s ign i f i can t q u a n t i t i e s have been found in meteori tes and in c e r t a i n p l a n e t a r y a tmospheres .

Nitrogen and phosphorus a r e p re sen t in a l l forms of l i fe , where they a r e e s s e n t i a l , and they cons t i tu te very rough ly about 3 % and 1 \ r e spec t ive ly of the to t a l weigh t . The r ema in ing p n i c t i d e s a r e not thought to be involved in l ife p rocesses in a n y s i g n i f i c a n t way, and become toxic when p resen t in more t h a n t r a c e amounts .

1.2 5

TABLE 1-1

Cosmic Abundance of Pnictide Elements

Nitrogen

Phosphorus

Arsenic

Antimony

Bismuth

N

P

As

Sb

Bi

6.6 x 10 4

1.0 x 10

4.0

0.25

0.14

♦Estimated num ber of atoms in the solar

system per 10 atoms of Si.

TABLE 1-2

Annual World Production of Pnictide Elements * (i980)

.6 Nitrogen

Phosphorus

Arsenic

Antimony

Bismuth

N

P

As

Sb

Bi

78

25 x 10

10~ tons/element 6

3.3 x 10 4

7.4 x 10 4.2 x 10

3

* Combined plus elemental forms

TABLE 1-3

Average Concentrations of Pnictide Elements (ppm)

N As Sb Bi

Sea Water

Crustal Rocks

Soil

Atmosphere

0.

20

2300

780000

5 0.07

1050

500

0.001

0.003

1.8

6

0.0001

0.0005

0.2

0.6

0.001

0.00002

0.2

0.2

0.0001

The human body normally contains not more than about 0.00003% by weight of arsenic, and the quantities of antimony and bismuth are even smaller. Nitrogen and phosphorus are found in almost all foods (Table 1.4). Concentrations of the heavier pnictides in the latter should not (and seldom do) exceed about lppm.

6 1.2

TABLE 1-4

Nitrogen and Pho

Potatoes

Apples

Oranges

Tomatoes

White Bread

Chocolate

Milk

Fish

Cheddar cheese

Process cheese

sphorus C

N

0.34

0.03

0.13

0.14

1.40

0.75

0.52

3.00

4.08

5.50

ontents. of

P·

0.06

0.01

0.02

0.02

0.10

0.23

0.09

0.20

0.52

0.80

Foods (wt %)

Carrots

Peanuts

Beef (lean)

Eggs

Macaroni

Spaghetti

Margarine

Beer

Rice

All-Bran

N

0.11

4.50

3.25

1.97

2.41

2.39

0.01

0.04

1.09

2.40

P

0.02

0.39

0.20

0.22

0.16

0.12

0.01

0.01

0.10

0.90

Phosphate Mineral Deposits More than 300 different phosphate minerals are known but only

those in the Apatite group occur in sufficient abundance and concentration to serve as commercial sources of the element.

The commonest igneous apatite deposits consist mainly of Fluorapatite, Ca io(P04 ) eF 2 . but Chlorapatite, Ca10 (P04 )6 Cl2 , and Hydroxyapatite, Ca 1 0 (P 0 4 ) 6 (0 H) 2 , are also found. Apatite occurs mostly as a sedimentary deposit which is named Phosphorite or 'phosphate rock' . Most phosphorite is believed to be of marine origin. It is mainly amorphous and is almost always found associated with calcium carbonate. Collophane is a term sometimes used to describe varieties of cryptocrystalline phosphorite which are fine grained and optically isotropic.

Varieties of carbonated apatite whose formulae may be represented as

C a i 0 - x ( P 0 4 > 6 - x ( C ° 3 ) x ( F ' 0 H ) 2 x */ 1

are often known as Francolite (F>QH) or Dahllite (0H>F). Up to 25% replacement of PO 4 by CO 3 is , however, sometimes found, and replacement of up to 10% Ca by Mg can occur. Other common impurities in phosphorites are iron, alumina & silica, but a wide variety of other metals, including uranium are found in trace amounts(Chapter 2.1).

The largest and most important phosphorite deposits are found in Morocco (Khouribga, Youssoufia, Essaouria, Bu Craa) , USA (Florida, N Carolina, Western States), USSR (Khazakstan, Ukha Gol), China (Yunnan) and Tunisia (Gafsa). Important deposits also occur in Togo, Senegal, South Africa, Algeria, Jordan, Egypt, Turkey, Israel , Brazil, Australia (Queensland), Nauru and other Pacific is lands.

Apatite also occurs (less abundantly) as igneous phosphate rock which is highly crystall ine and much purer than sedimentary phosphorite* Commercially important igneous rock formations of crystalline fluorapatite are found in the Kola peninsula of USSR, South

1.2 7

Africa (Palabora), Brazil (Jacupiranga) and Finland, but these all at present account for less than 15% of the world total of mined apati te .

There are substantial deposits of aluminous phosphates, but satisfactory development of these has not yet taken place. The aluminous phosphates include such minerals as Augelite, Crandallite, Millisite and Wavellite (Table 1.5). Aluminous phosphates are found in Senegal, Uganda, Nigeria, Brazil, par ts of Siberia and in small quantities in association with many apatite deposits . l t is believed that such minerals as Crandallite and Wavellite may have been formed by weathering, under acid conditions, of apatite deposits in the presence of aluminous clays.

*The term 'phosphate rock' sometimes describes any type of deposit.

TABLE 1-5

Mineral Phosphates of Galcium and Aluminium

Fluorapatite

Chlorapatite

Hydroxyapatite Ca

Brushite

Monetite

Whitlockite

CalO(P 04)6F2

Ca10(P04>6C12

10(PV6(0H)2

CaHP042H20

CaHPO

Ca3(P04)2

Berlinite

Variscite

Augelite

Crandallite

Millisite

Wavellite

A1P04

A1P04.2H20

A12P04(0H)3

Al3Ca(P04)2(0H)52H20

Al6NaCa(P04)4(0H)g3H20

A13(0H)3(P04)25H20

In some localities, apatite occurs as 'nodules' on the sea bed, as phosphatic limestöne, and in various other forms. These varieties are generally too dispersed or have such a low concentration of apatite that their commercial exploitation does not at present assume much importance. The extent of deep-sea-bed apati te has been little explored and available information relates mostly to shallow offshore regions. These regions include Portugal, Morocco, South West Africa, Peru, Chile, Southern California, Eastern New Zealand and Eastern USA. Phosphorite is present in submerged mountains, mostly in the N Pacific.

There are substantial reserves of oil-shale phosphorites in Israel , Jordan and the Eastern Mediteranean, but these are not economic to exploit with present technology.

A selection of known mineral phosphates are listed in Appendix IX. While some of the listed varieties are widely distributed, others are quite rare and. occur only in small veins or pockets,or as minor constituents of other commoner phosphate mineral deposits. Crandallite, Millisite and Wavellite (Table 1.5) e .g . , are found in Florida phosphorite deposits.

Apart from those in the apatite group, the best known phosphate minerals are Autunite, Crandallite, Lazulite, Millisite, Torbernite, Turquoise, Vivianite and Wavellite.

Some phosphate minerals have closely related internal structures and these can be placed in isostructural groups. Members of such groups can sometimes form mutual solid solutions over a range of composition. In addition to the Apatite group, which includes a large

8 1.2

number of mine ra l s and syn the t i c p roduc t s (Chapter 3.2) the re a r e e . g . those in Table 1.6.

Phospha te m ine ra l s , l ike s i l i c a t e m i n e r a l s , a r e found with a g r e a t v a r i e t y of c a t i o n s . Unlike the l a t t e r , which con ta in numerous types of s i l i c a t e a n i o n s , almost a l l phospha te mine ra l s a r e o r t h o p h o s p h a t e s . c o n t a i n i n g the PO43" ion. Non-phosphorus an ions such 10p

0* soi SiCV etc may a l so be p r e s e n t in a s LK- OH", ,F - Cl" t he se (Chap te r 3 . 2 ) .

The most f r equen t ly occu r r i ng ca t ions l ie in the o rde r : Fe5 + > Fe 2 + > A l 3 + > Ca 2 + Mn 2 +

Phospha te mine ra l s with p a r t i c u l a r ca t ions a r e l i a b l e to occur in the v ic in i ty of other (non-phosphorus ) ores c o n t a i n i n g the same c a t i o n s . Torbe rn i t e , Viv ian i t e , and Pyromorphi te e . g . , a r e found in the v ic in i ty of impor tan t u r an ium, i ron and lead ores r e s p e c t i v e l y .

Cer ta in r a r e phospha te mine ra l s such a s Monazite, (Ce ,Li ,Th)P04 and Xenotime , ÕÑÏ4, a r e impor tan t sources of r a r e e a r t h e lements . Monazite, which is mined in Braz i l , T r avanco re and A u s t r a l i a , i s an impor tan t source of t h o r i a , TI1O2. One commercial source of l i th ium is from LiNaP04, which is p resen t in some n a t u r a l b r i n e s . Phosphor i te i tse l f i s a p o t e n t i a l l y v a l u a b l e source of Uranium (Chapter 2)

Mineral specimens a r e f r equen t ly impure , the impur i t i e s be ing p resen t e i the r a s mechan ica l ly s e p a r a b l e m a t e r i a l , or in sol id solut ion in the c r y s t a l l i n e l a t t i c e . (Chapter 3.2) Many iron phospha t e s conta in a l i t t l e manganese , and manganese p h o s p h a t e s a l i t t l e i r o n , i n sol id so lu t ion .

TABLE 1-6 Isostructural Groups of Phosphate Minerals

Lazulite Group

Lazulite MgAl2(P04)2(0H)2

Scorzalite FeAl (P04) (OH)

Barbosalite Fe"Fe^'(P04)2(0H)2

Laueite Group

Laueite

Gordonite

MnFe£'(P04)2(0H)28H20

MgAl2(P04)2(0H)28H20

Paravauxite FeAl (PO ) (OH) .8H20

Montgomeryite Group

Montgomeryite Ca MgAl (PO ) (OH) 12H 0

Kingsmountite Ca^FeAl,, (PO J^(OH) 12Çï0 4 4 4 b 4 d Zodacite Ca4MnFe4(P04)6(0H)412H20

C a l c i o f e r r i t e Ca.MgFe .(PO J,.(OH) Ë2Ç„0 4 4 4 b 4 d

Triplite Group

Triplite

Zwieselite

Wagnerite

(Mn.Fe) PO F

(Fe,Mn)2P04F

Mg2P04F

Triploidite (Mn,Fe)pPO OH

Torbernite Group

Torbernite Ca(U02)2(PO ) 10H20

Uranocircite Ba(UO ) (PO ) 10H 0

Autunite

Bassetite

Saleeite

Ca(U02)2(P04)210H20

Fe(U02)2(P04)210H20

Mg(U02)2(P04)210H20

Crandallite Group

Crandallite CaAl H(POJo(0H) 6 Ad o

Goyazite SrAl3H(P04)2(0H)6

Plumbogumite PbAloH(P0Jn(OH)fi ô Ad. b

Gorceixite

Florencite

BaAl3H(P04)2(0H)e

CeAl3(P04)2(0H)6

1.2 9

Phosphoferrite, (Fe,Mn)3(P04)23H20, (Fe > Mn) e.g. can be compared with isostructural Reddingite, (Mn,Fe)3 (P04 )2 3H2 0, (Mn>Fe). Graftonite, (Fe,Mn,Ca,Mg)3 (P04 )2 (Fe,Mn > Ca,Mg) is isostructural with anhydrous Fe3(P04)2 and Mn3(P04)2.If weathering of Fe/Mn phosphates occurs, Fe2+ *Fe3+ oxidation takes place more readily than Mn 2+ >Mn 3 + .

Small amounts of impurities are not necessarily indicated in the chemical formulae which are frequently given in idealised form. Well-formed crystalline minerals are more likely to be free of impurities than microcrystalline and semi-amorphous varieties.

Phosphate minerals are usually identified and characterised by powder X-ray diffraction and refractive index determination, and supplemented by chemical analysis where necessary.

Many phosphate minerals can be prepared and crystallised as pure laboratory chemicals. Some of these 'artificial' minerals are of considerable industrial and biological importance and are better known under their chemical names (Chapters 3.2, 3.5, 6.1). Apatite Mining Development

Phosphorite was first mined in Suffolk, England, in 1847 and soon after in a few other places in Europe. Igneous apatite was first mined in Norway in 1851.Operations have long since ceased at most of these sites, however, and overall European production is now very small.

In North America phosphate rock was first mined in Ontario in 1863 and in South Carolina in 1867. These sources were soon superceeded by the discovery of the much larger deposits in Florida which have been exploited to an enormous extent from 1888 onwards. Production in Tenessee started in 1894 and in the Western states (Idaho, Utah, Montana and Wyoming) in 1906.

Since the beginning of the twentieth century limited amounts of phosphate rock have been exported from various parts of the Pacific, particularly from Nauru, Christmas Island and Ocean Island. Some of these sources are now almost exhausted.

Mining operations commenced in Algeria and Tunisia about the turn of the century, but production from the huge Moroccan deposits did not start until 1921.

The USSR commenced production in the Kola peninsula about 1930, although some mining had started earlier in acquired territories of Bessarabia and Estonia. Production and Reserves

World phosphate rock production has now surpassed 160 ÷ 106

tons p.a.(^50 ÷ 10 6 tons as P205) , having tripled over the last 40 years (Figure 1.5). About 90 % of this is put to fertilizer use. Total known apatite reserves have been estimated to last, at this rate of consumption, for a period of up to 1000 years.

Some estimates are much less optimistic, however, when the present explosive growth of world population is taken into account. If this expanding population is to be adequately fed, supplies of phosphate rock for fertilizers, at economic levels, may well become crucial within a century or so.

Today, Morocco, USA and USSR have become the largest producers and probably hold at least 75 % of the world's reserves between them. Although the USA (chiefly Florida) is at present the largest producer of sedimentary phosphorite ( % \ world total), Morocco remains the world's largest exporter and probably has the largest phosphorite reserves which are economically workable with present technology .

10 1.2

^ F > 10 tons/p.a.

• 106-107 - »

O New producing areas

F igure 1.1 World Centres of Phosphate Mining

WESTERN STATES

N . CAROLINA

TENNESSEE

FLORIDA

Figure 1.2 Major Phosphate Deposits in USA

1.2

Figure 1,3 Major Phosphate Deposits in Morocco

Figure 1.4 Major Phosphate Deposits

(a) South America (b) Africa

Open circles = igneous, Filled circles = sedimentary

11

12 1.2

TABLE 1-7

Phosphate Rock Production 1989 (*10u tons)

USA

USSR

Morocco

China

Tunisia

Jordan

Israel

Togo

48.9

34.4

24.4

15.5

6.1

6.0

3.9

3.4

South Africa

Senegal

Syria

Brazil

Nauru

Iraq

Algeria

Egypt

2.9

2.3

2.2

1.5

1.5

1.3

1.2

1.1

TABLE 1-8

Exports of Phosphate Rock (xlOu tons)

Morocco

USA

Jordan

USSR

Togo

14.9

11.3

4.7

3.7

2.8

Israel

Senegal

Nauru

Xmas Isle

Tunisia

2 . 1

1 .5

1 .4

1 .3

1 .1

Europe (EEC) remains the largest importer of phosphate but is now a negligible producer. China has expanded production and is endeavouring to become self sufficient. India is also increasing phosphate rock production, but the remainder of Asia appears to have insufficient resources to meet its requirements now or in the forseeable future

Figure

Annual

Output

1 • 5 World

of

Phosphate

Mined

Ores

1940 1960 1980 2000

Reliable figures for many phosphate rock reserves are difficult to obtain since conflicting data have been published and estimates are continually being revised. If only present economically workable

1.2 13

deposits are considered, Moroccan (including Western Sahara) reserves (20,000 - 50,000 million tons) exceed the combined resources of USA, USSR and China. On the other hand this is probably not true if the less accessible but enormous 'phosphoria ' deposits of Western USA and the lower grade apatite-nepheline Kola ores of the USSR are taken into account. Furthermore, the extent of the recently discovered Australian (NW Queensland), Peruvian and Venezuelian deposits has not been fully reported, although their commercial exploitation has begun.

Although many factors should be taken into account, abundant natural resources can be considered as falling into one of four classes of commercial viability (Table 1.9)

TABLE 1-9

Grades of Apatite Deposits

(1) Economic

(2) Sub-economic

(3) Non-economic

> 20 % P o 0 c

5-20 % P 2 0 5

1-5 % P 2 0 5

(4) N o n - p h o s p h a t e 0 . 1 - 1 % P O 2 5

Florida & Moroccan sedimentary phosphorites, Kola & Palabora crystalline igneous apatites.

Western USA Phosphoria, USSR nepheline apatites.

Low grade ores, phosphatic limestone.

Widely distributed apatite in almost all igneous rock

Estimates of total world reserves, based on (a) and (b) , have been put in the range 50-200 billion tons.

Workable apati te deposits occur mostly near the ea r th ' s surface in s t ra ta varying from a few inches to over thirty feet. About 80% of the world's phosphorite is obtained by open-cast mining methods. Concentration of the mined phosphorite is invariably carried out on site and various grades of beneficiated ore are usually commercially avai lable . Flotation processes are most often used, and by these means low grade ores ( ^10% P2O5) can sometimes be upgraded to about 30% P 2 0 5 .

The so-called 'weathered' deposits are usually the most satisfactory to work since the action of weathering is to remove much of the cementing carbonates, with a consequent softening of the phosphorite and increase of its P content. Weathered rock is usually more reactive and therefore more satisfactory than other grades for use in chemical plant .

Over three quarters of the world's phosphate rock production is converted into orthophosphoric acid by the 'wet' process (Chapter 3.1).Almost all of this is used to make fertilizers and less than 5 % is used to make other phosphorus compounds. Many of the latter are made via the element itself, which is obtained directly from apatite by the electric furnace method (Chapter 2.1).

Phosphate rock is sometimes used directly, in finely ground form as a fertilizer, or as an animal feed supplement if the fluorine has been removed by prior heat treatment (Chapter 6.8).

m 1.2

TABLE 1-10

Average Contents of Phosphate Sources .(wt % P205).

Fluorapatite (pure)

Kola (igneous)

Nauru (phosphorite)

Florida (sedimentary)

Khazakstan

42

40

39

35

23

Morocco (sedimentary)

Tunisia (sedimentary)

West USA (phosphoria)

Queensland

Venezuela

35

28

18-30

16-30

20

Kola (nepheline)

Bone Meal

Basic Slag

Guano

California (sea bed)

12-20

20

10-22

12

30

These figures do not necessarily apply to all the deposits in the

particular location named. Concentration and blending is usually

carried out to obtain commercial grades. Most available grades ex

Florida or Morocco contain 27-35 % Po0._. 2 b

The term 'phosphorite' is usually used for all sedimentary rocks

which contain at least 20 % Pn0c. If the content is below this 2 b

f i gu re , the mineral i s termed a 'phosphat ic o r e ' .

Alternative Sources of Phosphorus An alternative source of phosphorus is guano - a natural

deposit formed from decaying bones and excreta from fish-eating birds . Bird dung was employed by the Carthaginians as early as 200 BC in order to improve crop yields. Guano deposits are found in Chile, Peru, Mexico, Seychelles, the Arabian gulf and elsewhere, but they account for less than 2% of the world phosphate production. It is used almost exclusively for fertilizers.The Nauru and Christmas Island phosphorite deposits may be guano in origin (Table 1.8) It is believed that rainwater can carry soluble phosphate from Guano then trickle over rocks and interact with them to form phosphatic layers (e.g. phosphatised coral rock). Guano, mainly from Peru, assumed greatest importance about the middle of the nineteenth century, shortly before the phosphate rock industry began to establish itself.

A very minor source of phosphorus is basic s lag. This is the waste product from blast furnaces operating on iron ores with a significant phosphorus content. Basic slags contain tetracalcium phosphate, Ca3(P04)2 .CaO, and Silicocarnotite, Ca3 (P04 ) 2 .Ca2 Si04 , and they are applied directly as fertil izers.

Animal bones, which were recognised as a source of phosphorus at an early date, are still used after conversion to 'bone meal' by grinding, or to 'bone ash ' by calcining. Such products are rich in calcium phosphates and are used as fertilizers or as supplements to animal foodstuffs. Bones are still preferred as the source of calcium phosphate in the manufacture of the best bone china.

Human and animal excreta both contain phosphates. It has been estimated that the amount of phosphorus daily urinated by the people of the world is more than double that consumed by detergents.

Apatite has been found in lunar dust, but outside the earth

1.2 15

phosphorus has usually been detected in reduced forms. These include iron phosphides in meteorites and phosphine in planetary atmospheres.Spectroscopic molecules such as PO, PN & PC have been detected in interstel lar space (Chapter 2.5). Natural and Artificial Cycles of Phosphorus

The overall natural and artificial cycles involving phosphorus may be represented approximately as in Fig 1.6 . No appreciable amounts of gaseous phosphorus compounds are involved, and these cycles are restricted to the lithiosphere and the hydrosphere. Rainwater contains % 0.001% P. Originally, it can be supposed that sufficient phosphorus became available for life processes through the slow solubilisation of phosphate mineral deposits. The resulting distribution of the element on land and in the seas then enabled the initiation and development of life to take place. Subsequent death and decay of these organisms ensured a return of phosphorus to the system.

In the ea r th ' s crust, phosphorus takes second place to carbon, and in comparison with all known elements it takes about twelfth place in natural order of abundance. The atom ratio of 1P:15N which exists in the oceans is not greatly different to that found in living organisms. The avai labl i l i ty of soluble phosphate from weathering of apatite-containing rocks may ini t ial ly have been the rate-determining factor in early life development. In most ecological systems the phosphate content is the limiting factor for growth.

J SEDIMENTARY DEPOSITS

1 (Phosphorite)

> A

IGNEOUS APATITE

1

f /

FERTILISERS, 1 DETERGENTS, f Manufactured P compounds J

1 Prehis 1— geolog

uplift

- •>

GENERAL IGNEOUS ROCKS (Low cone apatite)

> Rain.weathering

f

SOIL

Precipitation by Ca >

toric L c a 1 ' s ^

\ Rivers

Death ' Decay

v s

< etc

/

OCEAN

Apatite

Deposits 'Sf,

ANIMAlJ LIFE j

> V

PLANTS FOOD

Decay of ^"Organisms

Figure 1.6 Natural and Artificial Cycles of Phosphorus

Nearly all igneous rocks contain some phosphate, even if only ^ 0.1% (0.2% P2 O 5 average for l i thiosphere), with most of it in the form of apat i te . Sedimentary rocks generally contain rather less (^0.1% P2O5 on average) . Sedimentary phosphorite is believed to have originated from the widely dispersed apati te mainly in igneous rocks.

16 1.2

Weathering and leaching processes of millions of years ago led to the transfer of the phosphate to rivers and to the oceans where it was concentrated in shells, bones and marine organisms which were deposited on the ocean floor. Subsequent uplift and other geological movements led to these accumulations becoming dry land deposits. However, these sedimentary deposits together with the concentrated igneous rock formations, which form the viable commercial sources of the element, represent only a fraction of the total phosphorus which is still present in widely distributed igneous form.

It is believed by some that this widely distributed phosphorus may have originated from nuclear disintegration of silicon in pre-biological ages (Chapter 13.6.) . Ocean Phosphate

The total amount of phosphorus in the oceans has been estimated to be * I01 1 tons . It arises principally from the P content (mostly as suspended matter) of inflowing rivers and to a lesser extent from the solubilisation of rocks. Some of the phosphate present in fish, algae etc is recycled but there will be a loss from bones and shells of dead species which tend to sink to the ocean bottom. 2_

The soluble inorganic P in seawater is present as HPCU " , H2P04" and P04

3" together with an abundance of Ca++ and ,of course NaCl.

Calcium phosphate is more soluble in seawater than in distilled water because of the presence of the sodium chloride and the effects of complex and ion-pair formation.At pH = 8.0, HPOA

2" ions form about 87% of the anionic phosphate species.

The soluble phosphate content of seawater varies seasonally and geographically, but generally increases with depth up to about 1000 metres. At greater depths it tends to remain constant at an average level of the order of 0.1 mg P/1. In this region any inflowing soluble phosphate is probably counterbalanced by slow precipitation of hydroxyapatite by the relatively abundant Ca+ + .Upwelling of water from the ocean depths in certain regions (e .g . off the coast of Peru), results in a local increase of phosphate concentration and the prolific production of marine life. Sediments on the deep ocean floor may well represent the most abundant source of phosphorus (see below). Phosphate in Lakes and Rivers

In lakes and rivers the soluble phosphate content is very variable and can be accompanied by phosphate absorbed on suspended clay particles as well as that present in fish, algae and other living matter.

Unlike in the ocean where phosphate is precipitated as a calcium salt , in lakes (and to a lesser extent in r ivers) , insoluble iron and aluminium salts may also be involved. Reactions are complicated and are affected not only by the composition of the imput water, but by climate and the numerous types of suspended matter which can already be present in the lake. Acid sediments favour the formation of aluminium and iron phosphates, but with neutral or alkaline conditions, calcium salts are likely to predominate. Distribution of Apatite

Precise quantitative estimates of world wide P distribution are difficult to make but presently available evidence suggests the total quantities lie in the order :

Ocean sediments » soil (low concentration) > ocean, r ivers lakes (soluble) > dry land deposits (mineable concentrates) > living matter

1.2 17

Phosphorus occurs in combinat ion with calcium in the most a b u n d a n t and widesp read minera l form, which i s a p a t i t e . B o t h elements a r e p r e sen t in almost a l l rocks and in the oceans . Both elements a r e p re sen t in n e a r l y a l l foods and i t i s p r o b a b l y no acc iden t t h a t they a r e u t i l i s ed by a l l an imal life (Chap te r s 11 & 12) .

The u n d e r s t a n d i n g of the p rocesses depic ted in F igure 1.6 remains far from complete. The en t i r e n a t u r a l cycle h a s to be reckoned in terms of mil l ions of y e a r s if the r e t u r n of phosphorus from the oceans to the l a n d is i n c l u d e d . This per iod s t a n d s in con t r a s t to the much shor t e r n a t u r a l cycles of n i t rogen and the other major life e lements . I t seems u n l i k e l y t h a t o v e r a l l equ i l ib r ium of the phosphorus cycle can be r e a c h e d , and the re is u s u a l l y cons idered to be an ove ra l l loss to the ocean d e p t h s . During the p r e sen t cen tu ry th i s loss h a s almost c e r t a i n l y been i n c r e a s e d by the widespread use of P in f e r t i l i z e r s , de t e rgen t s and other t e chn ica l p r o d u c t s . The q u a n t i t y of phosphorus r e a c h i n g the oceans from m a n ' s a c t i v i t i e s i s now of the same order a s t h a t a r i s i n g from n a t u r a l p rocesses and the full effects of these changes on the environment have yet to be a s sessed (Chapter 6 ) .

Arsenic is p r e sen t main ly a s a r s e n a t e in the t r a c e q u a n t i t i e s of the element which a r e found in seawa te r and in life c e l l s . Compara t ive ly l i t t l e i s known about the n a t u r a l cycle of t h i s element which may to some exten t be a s soc i a t ed with t h a t of p h o s p h o r u s . Trace q u a n t i t i e s may be n e c e s s a r y for some life p rocesses (Chapter 13). Eu t rophica t ion and Pol lut ion

Eu t roph ica t ion in l akes ( and sometimes in r i v e r s ) i s caused by the presence of undu ly h igh concen t r a t ions of n i t r a t e s a n d / o r phospha te s which encourage the excess ive growth of a l g a e * . Severe oxygen deple t ion of the l ake water can then r e s u l t from the in te r fe rence with the pho tosyn the t i c p roces s , caused by the reduced pene t r a t i on of s u n l i g h t , a s well a s from subsequen t decay of these a l g a e .

TABLE 1-11 Typical Nitrogen and Phosphorus Levels in Aqueous Systems (ppm)

Agricultural Drainage Water

Domestic Waste Water

Treated Sewage Effluent

Rain Water

Lake Water

River Water

N

10

40

30

1

0.3

5

P

1

10

5

0

0

1

Two of the most impor tan t fac tors c o n t r i b u t i n g to eu t roph ica t ion a r e the h igh n i t r a t e content of a g r i c u l t u r a l d r a i n a g e water and the

♦g rea t e r t h a n about 0.1 ppm of N or O.Olppm of P .

18 1.2

high phospha te content of domestic sewage .The f i r s t h a s a r i s e n from the i nc r ea sed use of n i t rogen f e r t i l i z e r s and the l a t t e r h a s been caused mainly by the i nc reased use of po lyphospha te d e t e r g e n t s . Human excrement a l so makes a l a r g e con t r ibu t ion to the phospha te content of domestic sewage , and f e r t i l i z e r s a r e r e spons ib l e for a s i gn i f i can t phospha te content of a g r i c u l t u r a l d r a i n a g e water.Some r e p r e s e n t a t i v e pol lu t ion leve ls a r e i nd i ca t ed in Table 1.11 »al though i t should be remembered t h a t these can v a r y c o n s i d e r a b l y with locat ion and with season .

F reshwate r l a k e s a r e u s u a l l y most s ens i t i ve to phospha t e pol lu t ion s ince they a l r e a d y have a n i t rogen content in excess of 15N:1P which is r e q u i r e d by l i v i n g o r g a n i s m s . On the other h a n d in the r e l a t i v e l y p h o s p h o r u s - r i c h ocean, the n i t rogen content i s be l ieved to be the l imi t ing factor to the growth of l i v i n g ma t t e r .

Eut rophied l a k e s can often be rec la imed if the phospha t e content of the inflowing water or sewage i s d r a s t i c a l l y r e d u c e d . This a l lows the e x i s t i n g phospha te level to slowly reduce i tself by n a t u r a l p r e c i p i t a t i o n and sed imenta t ion . Since a g r i c u l t u r a l d r a i n a g e wa te r s a r e more diff icul t to cont ro l , efforts a t phospha te reduc t ion have been concent ra ted mostly on sewage t r ea tmen t . Careful f i l t r a t i o n followed by p r e c i p i t a t i o n of h y d r o x y a p a t i t e with added lime, and then fur ther f i l t r a t ion th rough carbon beds , can remove up to 98% of the phospha te content of sewage wa te r . Effluents from sewage p l a n t s in Sweden and Swi tzer land a r e t r e a t e d with so luble aluminium or i ron s a l t s to p r e c i p i t a t e inso lub le aluminium or i ron p h o s p h a t e s . There a r e a lso va r ious b io - t r ea tmen t processes for the removal of P from w a s t e w a t e r s .

The compara t ive ly high n i t rogen and phosphorus contents of some sewage s ludges r ende r them s u i t a b l e for f e r t i l i z e r s (Chapter 6 .1 ) .

Re la t ive ly h igh phospha te l eve l s in d r i n k i n g water a r e qu i t e h a r m l e s s , but n i t r a t e s can be tox ic , g iv ing r i s e e . g . to methemoglobinemia in b a b i e s . The level of both elements shou ld , however, be kept down to avoid encou rag ing the growth of b a c t e r i a . There a r e u s u a l l y no problems in keep ing the r ema in ing p n i c t i d e s below toxic l eve l s (Table 1.12).

TABLE 1-12 Normal Pnictide Contents of Potable Water /çç_\

N P As Sb Bi

0.01-10.0 0.001-0.1 0.001-0.01 0.001 0.001

Futu re Outlook The use of n i t rogen and phosphorus compounds, p a r t i c u l a r l y a s

f e r t i l i z e r s , is l ike ly to i n c r e a s e c o n s i d e r a b l y th roughou t the next cen tury (Chapter 6 2 ) . This will be neces sa ry in order to feed the r a p i d l y e x p a n d i n g world popu la t ion , and the ecology of these two elements h a s become a subject of major impor tance .

Nitrogen supp l i e s a r e ob ta ined from the a tmosphere . The l a t t e r forms an i n e x h a u s t i b l e r e se rvo i r because of a r e l a t i v e l y shor t n a t u r a l cycle of the element invo lv ing cont inuous b a c t e r i a l p rocesses of f ixa t ion , n i t r i f i ca t ion and den i t r i f i ca t ion etc (Chapter 13) The e v e n t u a l deple t ion of r e a d i l y a v a i l b l e phospha te rock s u p p l i e s , on the other h a n d , seems not u n l i k e l y .

1.3 19

The b u i l d - u p . of p h o s p h a t e s in the oceans or on the ocean beds may, in a few c e n t u r i e s , make the ocean the most economic if not the sole convenient source of s u p p l y . As an a l t e r n a t i v e to the d i r ec t mining of sea bed a p a t i t e , however, i t may become poss ib le to develop spec ies of mar ine p l a n t s which could ob ta in t h e i r phospha t e d i r e c t l y from the ocean w a t e r s , t h u s removing the necess i ty for f e r t i l i ze r manufac ture a s we know i t t o d a y .

Toxici ty of Phosphorus Compounds Although n a t u r a l l y - o c c u r r i n g phosphorus compounds a r e almost

i n v a r i a b l y non- tox ic , known s y n t h e t i c p roduc t s show a very wide r a n g e of t o x i c i t y . Most i n o r g a n i c p h o s p h a t e s b a s e d on p e n t a v a l e n t phosphorus a r e among the safes t of a l l s u b s t a n c e s known to man ( u n l e s s , of course , toxic ca t i ons a r e p r e s e n t ) . They a r e e s s e n t i a l to n u t r i t i o n a n d a r e consumed in food and soft d r i n k s and a r e t aken in tonics and medicines (Chapte r 6 ) . The v a r i e t i e s employed in too thpas tes and de t e rgen t s a r e completely h a r m l e s s . No d e a t h s or i l l nes ses have r e s u l t e d from these p h o s p h a t e s be ing p r e s e n t in wa te r , foods or other commodities (Tab les 1.4 & 1.11).

Organophosphorus e s t e r s and i n o r g a n i c p h o s p h a t e s a r e widely d i s t r i b u t e d in l i v i n g t i s s u e s . On the o ther h a n d , some o rganophosphorus compounds with the element in p a r t i c u l a r chemical env i ronments , cons t i tu t e the most powerful poisons known to man, and can be u t i l i s ed a s ne rve g a s e s (Chapter 6 .11) . Many other o rganophosphorus compounds have v a r y i n g degrees of t ox ic i ty , a s have some i n o r g a n i c d e r i v a t i v e s (Chapte r 2 ) . T r i v a l e n t phosphorus compounds a r e often very toxic in small concen t r a t i ons e . g . white phosphorus P 4 and phosph ine PH 3 (Chapter 4) (Appendix V ) .

1.3 ATOMIC PROPERTIES

Phosphorus , symbol P, atomic number 15, atomic weight 30.97, ex i s t s a s the s t a b l e isotope 3 1P with a n u c l e a r sp in of i , and cons t i tu tes 100% of the n a t u r a l l y a b u n d a n t spec ies (Table 1.13). Six u n s t a b l e i sotopes a r e known (Chapte r 13.2) ,

TABLE 1-13 Atomic Data for Pnictide Elements

Element

Nitrogen

Phosphorus

Arsenic

Antimony

Bismuth

K

H Is

Symbol

N

P

As

Sb

Bi

2s

L

U

Atomic Number Atomic We

7

15

33

51

83

shell

H 2p

\\ 3s

14.0067

30.9738

74.9216

121.75

208.98

M shell

+1 +1 +1 3p

ight

ii

Stable Isotopes (wt %)

14N

JiP

/bAs m s b ^ B i

II

99.

100

100

57

100

II 3d

6 N 0.4

123Sb 43

II II

20 1.3

The e lec t ronic s t r u c t u r e of the phosphorus atom is I s 2 2s 2 2p6 3s2

3 p 3 with t h r ee u n p a i r e d e lec t rons in the outer 3p o r b i t a l s which a r e a v a i l a b l e for chemical b o n d i n g . Phosphorus can be formally t r i v a l e n t or p e n t a v a l e n t , u s i n g only t h r e e , or a l l five e l ec t rons in the outer M shel l to form s h a r e d e lect ron p a i r s with other atoms (Table 1.14).

TABLE 1-14 Electronic Structures of Pnictide Elements

Element

N

P

As

Sb

Bi

K

Is

2

2

2

2

2

L

2s

2

2

2

2

2

2p

3

6

6

6

6

3s

2

2

2

2

M

3p

3

6

6

6

3d

10

10

10

4s

2

2

2

N

4p 4d

3

6 10

6 10

4f

14

5s

2

2

0

5p

3

6

5d

10

P

6s 6p

2 3

In the g r e a t major i ty of i t s compounds, the element forms t h r e e , four or five cova len t l i n k a g e s to other atoms and among these , the four-connected a r e both the most numerous and t e c h n i c a l l y the most impor t an t . A much smal ler number of one, two and s ix -connec ted compounds a r e a l so known and in a few spec i a l cases the phosphorus may form some kind of chemical l i n k a g e with a s many a s ten close n e i g h b o u r s .

The chemis t ry of phosphorus g e n e r a l l y resembles t h a t of a r s e n i c much more c losely t h a n t h a t of n i t r o g e n . Whereas n i t rogen i s only t r i v a l e n t and forms covalent l i n k a g e s to not more t h a n four o ther atoms, a r s e n i c , ant imony and bismuth have d o r b i t a l s a n d , l ike phosphorus , can exh ib i t t r i or pen t a va lency and form five and s i x - coo rd ina t ed compounds.

Nitrogen often forms mul t ip le bonds whereas the r ema in ing pn ic t ide elements have less tendency to do so . Phosphorus , in p a r t i c u l a r , h a s a much g r e a t e r tendency t h a n n i t rogen to c a t e n a t e and form cont inuous c h a i n s of s ing le bonds i . e . - P - P - P - P - . All pn i c t i de elements except n i t rogen occur in polymer ised form.

There a r e formal s i m i l a r i t i e s , bu t s t r u c t u r a l d i f fe rences , between va r ious simple N and P compounds. The l a t t e r u s u a l l y ex i s t in more h i g h l y polymerised form, and the co r r e spond ing As compounds a r e u s u a l l y i s o s t r u c t u r a l :

E N=N

V - N ; /°

N - O - N o x o

6

1.3 21

E 0 o V - P — C M

II I

R . E O O R-îK

R — N = N — R

- P — 0

II 0 J

Some se r i e s of simple pn i c t i de compounds show a sys temat ic change in p r o p e r t i e s :

As Sb Bi

Hydr ides EH3

Oxides E2O3

Halides EC1,

l ess s t a b l e

more b a s i c

more b a s i c

Moving from P to Bi, the bo i l ing po in t s of the EH3 h y d r i d e s d e c r e a s e , while those of the EX3 h a l i d e s i n c r e a s e . Above -102°C, PF3 is a s t a b l e g a s , while B1F3 i s an ionic so l id , mp = 725°C. The t r i o x i d e , P2O3, i s a c i d i c , whereas AS2 O3 is only weakly a c i d i c , and Sb2Û3 and B12O3 a r e b a s i c . E l ec t ronega t i v i t y

Nitrogen is the most e l ec t ronega t i ve p n i c t i d e element (Table 1 .15)and, l ike phosphorus , a non-me ta l . The most e lec t ropos i t ive element, b i smuth , on the o ther h a n d , i s a t y p i c a l metal in some of i t s b e h a v i o u r . Arsenic and ant imony a r e i n t e rmed ia t e in c h a r a c t e r and a r e sometimes r e fe r red to a s ' m e t a l l o i d a l ' .

more meta l l ic

<r an ions formed more e a s i l y

As Sb Bi

ca t ions formed more e a s i l y

TABLE 1-15

Electronegativities of Elements

H 2 . 1

C " 2 . 5

S i 1 . 8

Ge 1 . 8

N

3 . 0

P 2 . 1

As 2 . 0

0

3 . 5

S 2 . 5

Se 2 . 4

F 4 . 0

Cl 3 . 0

Br 2 . 8

22 1.3

The tendency of group V elements to form simple positive cations increases with increasing atomic weight. This is indicated by the ionisation potentials which become lower as the atomic weight increases (Table 1.16). Conversely, the formation of simple negative anions occurs more readily in compounds of the lighter pnictide elements. All pnictide elements form polyanions (Chapter 2.2).

TABLE 1-16

Ionization Potentials for Group V Elements (eV)

x+

x++

x + + +

x + + + +

x + + + + +

N

1 4 . 5

2 9 . 5

4 7 . 4

7 7 . 0

97 .4

P

10 .9

1 9 . 6

30 .0

51 .0

6 5 . 0

As

1 0 . 5

2 0 . 1

2 8 . 0

4 9 . 9

6 2 . 5

S b

8 . 5

1 8 . 0

2 4 . 7

4 4 . 0

5 5 . 5

B i

8 . 0

1 6 . 6

2 5 . 4

4 5 . 1

5 5 . 7

TABLE 1-17

Characteristic Radii of Pnictide Elements

N

P

As

Sb

B i

5+ r

0 .11

0 .34

0 .47

0 .62

0 .74

r c o v a l e n t

0 .74

1.11

1.21

1.41

1.46

3 -r

1.71

2 .12

2 .22

2 . 4 5

-

mel

-

1.18

1.40

1.61

1.82

b a l l i c r

van d e r Waal

1 . 5

1 . 9

2 . 0

2 . 2

-

Nitrogen is sufficiently electronegative to form strong hydrogen bonds whereas those involving phosphorus are very weak and they do not appear to be formed at all by arsenic, antimony and bismuth.

The electronegativity of nitrogen (3.0) equals that of chlorine, and is exceeded only by that of oxygen (3.5) and fluorine (4.0). The electronegativity of phosphorus (2.1) is equal to that of hydrogen, is greater than that of silicon (1.8), but less than that of carbon (2.5) or oxygen. Basic Stereochemistry

Possible symmetrical arrangements of up to 9 bonds around a central atom are depicted in Fig 1.7. All of these have been found in phosphorus compounds. The common stereochemical configurations adopted by 3,4,5 & 6-connected phosphorus are i l lustrated by the compounds ( l a - f ) .

ci \C1 ci

0

II

ci f \ a

C l

ci la ci

0 II

0

C l I 1 C l

C l — Ñ ^

1 C1

Cl

C l c i ^ L · ci c i ^ i ^ c i

C l (1)

(a) ( b ) ( c ) ( d ) ( e ) ( f )

1.3 23

s*^ ^f s*r- <^ η^ angular (2) trigonal

planar Q )

pyramidal(3) tetrahedral(^) 4 . » , /ex trigonal KZ>) bipyramidal

tetragonal pyramidal (5) trigonal (6)

prismatic tetrakaideca-

hedral (9)

Figure 1.7 Basic Geometry of Bonding Systems

The pyramidal structure of phosphorus trichloride, PC13 typifies tr ivalent phosphorus compounds, while tetrahedral phosphoryl chloride, P0C13, the ions PC14

+ & P043~ together with trigonal bipyramidal

phosphorus pentachloride, PC15, typify the spatial arrangements adopted by penta valent phosphorus compounds. The hexachlorophosphate anion, PCle" , based on an octahedral bond configuration represents a rather smaller group of compounds . There are a few stable 5-connected compounds which have a tetragonal pyramidal rather than a trigonal bipyramidal arrangement of bonds.

/ V -RO 0 N0

vN Ê(Ë>

(2)

OR

Or thophospha te e s t e r s (2) de r ived from ( Id ) (where the o r g a n i c g roup , R, can be very complex) a r e p a r t i c u l a r l y a b u n d a n t and impor tan t in a l l l ife spec ies .

Cl ^ 0 F3C N-CF2 Cl Cl Ph ^ P h (3)

One and two-connected P compounds a r e of much less common occurrence t h a n 3,4 or 5-connected compounds, a l t hough many 2-connected v a r i e t i e s have been syn thes i s ed over the p a s t two decades Examples of 2-connected P atoms a r e provided by molecules such a s C1-P=0, F2C=PCF3 and p h o s p h i n i n e , C5H5P. There a l so ex i s t ionised species such a s PC12

+ ca t ions and Ph2 P " an ions (3) .

P=rCH P==P P==N (4)

Examples of compara t ive ly r a r e 1-connected P atoms a r e p rov ided by PCH, P2 and v a r i o u s u n s t a b l e spec t roscopic molecules such a s ÑÇ,ÑÍ and PO (4)

24 1.3

ο—ΡΓ C 1 — P C ( 5 )

^ Ï X) (a) (b)

The existence of a stable planar monometaphosphate ion P0 3~(5a) seems doubtful although it probably has a transient existence in some reactions (Chapter 13.5). On the other hand, CIPO2 molecules (5b) have been isolated. As an alternative to the octahedral arrangement in the PCI 6" anion the P atom can have 6-fold trigonal prismatic coordination as in Fe2P, 8-fold cubical coordinaton as in Ir2 P or 9-fold tetrakaidecahedral coordination as in Fe3P. These and other schemes of coordination, up to 10-fold, are found amongst metal phosphides (Chapter 2.2), although the bonding may not be of a conventional covalent type (see below). In certain organometallic compounds single P atoms are also found with high coordination numbers of metal atoms. In [Os6(CO)ie P] " and [Rhio P(CO)22]3" e .g . , the P coordination is trigonal prismatic and cubic antiprismatic respectively although some ionic bonding may be involved.

Electronic Structure In terms of the classical octet theory, the electronic

configuration in pyramidal and tetrahedral phosphorus compounds is completed by an outer shell of eight electrons as indicated in (6).

(6)

In trivalent compounds each 3p electron pairs with one from the covalently attached atom, which together with the 'unshared ' 3s electrons makes up an outer octet around the P atom. Although individual electrons are indistinguishable, the shared ' lone-pair ' characterises tr ivalent phosphorus compounds and generally confers upon them a high degree of chemical reactivity. They often have a strong tendency to polymerise or oxidise and become pentavalent (7)

2PC13 + 02 ^ 2P0C1 (7)

In phosphoryl compounds such as P0C1 3 the covalent bond to the oxygen atom is often regarded as formed by donation of the 'lone pa i r ' electrons from the P atom. Such donation confers semi-polar or part-ionic properties on the bond and it can be written as P—0"" , P-*0 or as P = 0 , the 'double' bond completing the formal pentavalency of the central phosphorus atom.

By acquiring three extra electrons as in the very stable orthophosphate anion, PCU3', the P atom can form donor-type P-»0 linkages, while if an electron is lost from the P atom, four single covalent bonds are formed as in the tetrachlorophosphonium cation, PCl 4

+ . (6 ) .

rci: X · · ·

χ Ρ χ θ ; • X · «

:ci:

:ci: x· . .

: c i x P S o : X · . .

:cn

. . 3 -

: o : • · X · . .

: o x P î o : . . x . . .

:o :

+

:CL#

. . . x · . : c i ?PxCi:

X ·

:ci:

1.3 25

ci-Ë·. . · · . F . . . " 3-•9\+p·?.·' ·>.+ *· + * xx xx p " . · ·.*·.+ - + * ; χ ρ χ î p i H (8 )

ici!" t e r ; F + F . H • · · · ( a ) · ·" ( b ) ( c ) ( d )

In trigonal bipyramidal compounds such as PC15 , an outer shell of ten electrons is involved (8a), while in octahedral configurations such as PF6~, a negative charge is acquired and the outer shell probably contains twelve electrons(8b). The phosphide anion P 3 " probably exists in some metal phosphides and this will presumably be based on a completed octet of electrons (8c).A similar situation occurs in the phosphide PH2" anion (8d).

H H

H H • x · χ

x P ΟP .' • X · Χ

H H (a)

:o: ïoï • · · X » X X X

Jo" P x P : oî · · · χ · χ χ χ

: o : s o x X · «x H H [b)

OH OH 1 1

CX-P—P-*0

OH ΤH l e )

H3C F

H.C—P-^P~-3 / »NF

H C F Γ

(9)

Cd l

When trivalent phosphorus atoms link together as in diphosphine, P2H4 (9a), each P atom contributes an electron to form the single covalent bond. A similar situation exists with pentavalent derivatives such as hypophosphoric acid, H4P2O6 which can be represented as in (9b). The phosphoryl bonds in the compound are donor-type as in POCI3 above, with two electrons being provided by each P atom to complete the formal octet around the 0 atoms. It is , however, usually more convenient to represent the electronic formulae as in (9c,d)Examples 0f donor-type P-*P linkages are known but these are very r a re . In Me3P-*PF5, both electrons for the bond are provided by the same P atom (9d)

Whereas electrons in excess of the required octet are provided in PCI5 and PClô" (8), the outer valence shell may contain only six electrons in molecules such as (lOa-c), the phosphenium cation (lOd) or the (hypothetical) phosphinidene )10e).

X *

sei: *x

• * :P X X

:oi *>

; C I ; P * Ο O ; P . p . . p . (a ) '& ^ *Ä" (O (d) (e)

(10) Oxidation States

The oxidation state or oxidation number is a somewhat artificial concept, but it can be defined as the number of electrons that must be added or subtracted from an atom in its combined state to convert it to the elemental form.

In its compounds phosphorus can be considered to exist in various oxidation states which are related to the number of attached oxygen atoms. If tr ivalent phosphine, PH3, is taken as the lowest oxidation state of - 3 , the higher oxidation states are obtained by adding +2 for each oxygen atom which is attached. The attachment to P of OH, halogen X, or CR3 (R= H,Me,Et etc) requires the addition of +1 to restore it to the equivalent elemental s ta te . If the P atom is linked to another P atom, as occurs in some diphosphorus compounds, this leads, by definition, to oxidation states of +2 and +4. These rules apply to the great majority of phosphorus compounds and some typical formal oxidation states are given in Table 1.18.

26 1.3

TABLE 1-18

Oxidation States of Phosphorus Compounds

■3

2

1

PH3

H / H

P-P. H7 \

R \ R-j>=0 R7

PR3 PX3

\ /R

R7 \

X-P=0

x7

p-p' X 7 \

phosphines

diphosphines

phosphine oxides

phosphoryl halides

white phosphorus

+1

+2

+3

H-P=0

H0s OH H-P-P-H 0^ ^0

.OH H-P=0

X0H

p( Ç ' OH

/OH R-P=0

X0H

RW° p

R7 \)H

OR P-OR X0R

H0-P=0 H-P; V

phosphenous hydride hypophosphorous acid phosphinic acid

hypodiphosphoric acid

phosphorous acid phosphonic acid phosphite esters

phosphenous acid phosphenic hydride

+4 Ç0÷ ^ÏÇ 0=F-P=0

Çè' N0H hypophosphoric acid

+5 HO H0-P=0

R0N

R0-P=0 RO^

Ç0÷ OH 0=P-0-P=0 HO' \ OH

phosphoric acid phosphoric esters diphosphoric acid

HO-P phosphenic acid

Phosphorus exists in nature almost exclusively in the +5 oxidation state. Whereas nitrogen is found in both the +5 oxidation state as ni trate , and in the -3 oxidation state as NH3 , phosphine, PH3 appears to be absent from biochemical and geochemical systems. This may be because of a much greater energy difference between the

13 27

-3 and +5 ox ida t ion s t a t e s for P t h a n i s the case with N. In i t s g e n e r a l chemis t ry , phosphorus h a s a g r e a t e r a f f in i ty for oxygen t han n i t rogen h a s , bu t the l a t t e r h a s a s t r onge r a f f in i ty for hydrogen t h a n p h o s p h o r u s . Arsenic compounds a r e known in a l l the ox ida t ion s t a t e s co r respond ing to those of phosphorus compounds.

TABLE 1-19 Valence Bond Structures for Phosphorus Compounds

A p

λ 3 σ 3

A λ " σ "

— P C I 5 „ 5 λ 3 σ

\y s\

λ ^ σ 6

P =

ë 2 ó 1

·—P==r i p s

λ 3 σ 2 > λ 3 σ 1

A λ α σ *

»P=» ■ — P s

ë 4 ó 2 I ë ^ ó 2

- P = r

5 „ 4 λ ^ σ λ 5 σ 3 ' λ 5 σ 3

= P =

\ 5 ó 2

7Γ I -

3p=, I

\pk V= V= λ 6 σ 5 λ 6 σ 4 λ ^ σ ^ \ 6 σ 3 \S σ 3 λ 6 σ 2

The use of conven t iona l va lence bond formulae s u g g e s t s t h a t a l l the a r r a n g e m e n t s shewn in Table 1.19 might be poss ib le for phosphorus compounds. The ëó nomencla ture i s used in t h i s t a b l e to s igni fy the covalency and coord ina t ion numbers r e s p e c t i v e l y .

The vas t major i ty of p r e s e n t l y known phosphorus compounds cor respond to those a r r a n g e m e n t s enclosed wi th in full r e c t a n g l e s , with the ë 5 ó 4 & ë3 ó3 v a r i e t i e s be ing the most common. The bond a r r a n g e m e n t s wi th in the broken r e c t a n g l e s r e p r e s e n t r a t h e r fewer compounds, while the r ema inde r a t p r e sen t r e p r e s e n t e i the r unknown, v e r y r a r e or only c o n t r i b u t i n g s t a t e s to a molecule.

Some 40 y e a r s ago , almost the whole of known phosphorus chemis t ry was d iv ided between t r i v a l e n t p y r a m i d a l U 3 ó3 ) , p e n t a v a l e n t t e t r a h e d r a l ( ë5 ó4) and a few p e n t a v a l e n t t r i g o n a l b i p y r a m i d a l ( ë 5ó 5) compounds ( l e ) - ( l e ) . Only s ince the 1960's have s ign i f i can t numbers of ë 3 ó 2 compounds and some of the other v a r i e t i e s l i s t ed in Table 1.19 been s y n t h e s i s e d .

The s y n t h e s i s of many of these l a t t e r compounds h a s c l e a r l y i n v a l i d a t e d the ' doub le bond r u l e ' which had come to be f a i r l y widely accepted by 1950. This r u l e s t a t e d t h a t the formation of double bonds between P and f i r s t row elements was impossible .

^ N S i M e (Me S i ) N P

3 2 ^ N S i M e

Ca)

= P , ^ C ( S i M e 3 ) 2

V C ( S i M e 3 ) 2 (11) ( b )

28 1.3

The sti l l uncommon ë 5ó 3 & ë6 ó3 arrangements are represented by compounds ( l la )&(l lb) respectively, while examples of ë4 ó2

compounds are provided by (12a,b) . The ë 5 ó 3 & ë4 ó3 type compounds are represented by (12d,e)and (12f) respectively. It also appears that ë6 a^compounds (12g) may exist .

Ph P=5^=spph

(a )

P h \ p / P h

k k

Vp/"'

(b)

V N f / ^ s

Ph \

OC—Mo~P~C(SiMe0)0

/ 3 2

(c )

" \ SiMe0 (d) (e) PPh ( f )

F 3 < V C F 3 II N

II 3 „ 3

N I! PPh„

(12)

(g)

Bond Orbitals The bonding in phosphorus compounds, as in other compounds, is

explicable in terms of the overlap of atomic orbitals in directions of high electron density. The basic geometries of the various atomic orbitals on the P atom are shown in Fig 1.8. In addition to the spherically symmetrical s orbital and the three orthogonal p lobes, there are 5 sausage-shaped d orbitals with the orientations shown.

Figure 1.8 Shapes of Atomic Orbitals

When the orbitals of the P atom overlap with the orbitals of other atoms, and there are sufficient electrons available to fill them, covalent bonds are formed. In general this overlap may be 'end on ' , corresponding to ó-bonding, or it may be ' s ideways ' , in which case it is called ð bonding (Fig 1.9).

1.3 29

o bonding 7Γ bonding

Figure 1.9 Schemes of Orbital Overlap

In the case of phosphorus the promotional energy 3s—»3d is small enough to allow the vacant d orbitals to part icipate in bonding and form hybridised orbitals which have special spat ial orientations. In the case of nitrogen and other first row elements with unfilled orbitals, the promotional energy 2s—>3d is too large for effective d bonding to take place (Fig 10).

t Nitrogen

-2eV

Phosphorus

..._L._. -2eV

9eV

3P- +1

Figure 1.10 Atomic Energy Levels for Nitrogen & Phosphorus

The ready avai labi l i ty of d orbitals in the case of phosphorus (and the heavier pnictide elements) accounts for many of their differences in chemistry compared to those of nitrogen.

The greater contribution of higher-energy d levels in the case of phosphorus leads to an effectively larger atom with reduced electronegativity and greater polarisabil i ty compared to that of nitrogen.

Another effect arises from the fact that the difference in size between the s and p orbitals is generally greater in second row elements than in first row elements (Fig 1.11). This reduces the amount of orbital overlap and resulting bond hybridisations and bond strengths are weaker than expected (Section 1.6). The effect contributes to the greater polarisabil i ty of all second row elements (Table 1.20).

30 1.3

TABLE 1-20

Dipole Polarizabilities of Elements

C

Si

Ge

11.8

36.3

41.0

N 7.4

P 24.5

As 29.1

2H

a tomic u e i g h t

Figure 1.11 Orbital Radii of First & Second Row Elements

The chemistry of phosphorus generally lies much closer to that of arsenic, than to nitrogen, and a given phosphorus compound often resembles its arsenic analogue in structure and many of its properties. Basic Schemes of ó -Bonding (single bonds)

The principal schemes of hybridised orbitals determine a basic system of ó - bonds as summarised in Table 1.21. TABLE 1-21

Hybridised Orbitals

Orbitals

2 P

sp

. 2 sp

3 P

3 sp

sp d z

sp dx2_

sPV2

2 ■y

No of

2

2

3

3

4

5

5

6

Bonds Angles (°)

90.

180

120

90

109 28'

90, 120

90

Configuration

angular

linear

trigonal planar

pyramidal

tetrahedral

trigonal bipyramidal

tetragonal pyramidal

octahedral

1.3 31

Table 1.22 i n d i c a t e s t h e c p o s s i b l e a r r a n g e m e n t s for ó - bonde d phosphorus in the t r i v a l e n t and p e n t a v a l e n t s t a t e s . The overwhelming number of a r r a n g e m e n t s in p r a c t i c e cor respond to those wi th in the broken l i n e s . Some r e p r e s e n t a t i v e examples of these have a l r e a d y been given in ( l a ) - ( l f ) .

TABLE 1-22

Single Bond Configurations of Phosphorus

«III nV

Trigonal

Tetrahedral

Trigonal bipyramidal

Octahedral

sp 6e

sp 8e

dsp 10e

2 3 d sp 12e

sK /f\

: p :

P C I ;

PCI.

PCI

42+ / p \

/N ·

"º

PCI.

PCI,.

PC1£

Since the apex a n g l e s in p y r a m i d a l phosphorus compounds a r e u s u a l l y n e a r e r to 100° t h a n 90° , the bonds a r e bes t desc r ibed a s mainly p 3 with some s p 3 c h a r a c t e r . In such cases the ' lone p a i r ' becomes involved in the bond ing to some d e g r e e . The ex ten t of l o n e - p a i r p a r t i c i p a t i o n d e c r e a s e s with the h e a v i e r members of the pn i c t i de g r o u p . This i s i n d i c a t e d by the d e c r e a s i n g H/X/H bond a n g l e s in the XH3 h y d r i d e s (Chapter 2 . 2 ) . Whereas the lone p a i r h a s no s e p a r a t e i d e n t i t y in t e t r a h e d r a l n i t r o g e n ( a n d ca rbon) compounds, the 6s e lec t rons do behave a s an i n e r t p a i r in the r e a c t i o n s of b ismuth compounds. An i n c r e a s i n g r e l u c t a n c e to form t e t r a h e d r a l s p 3 bonds is a l so i n d i c a t e d by the i n c r e a s i n g d i f f icul ty observed in forming q u a t e r n a r y ca t ions XR44" , on moving to the h e a v i e r elements of the pn ic t ide g r o u p .

The a x i a l ( a p i c a l ) bonds in t r i g o n a l b i p y r a m i d a l molecules a r e g e n e r a l l y s l i g h t l y weaker t h a n the e q u a t o r i a l bonds invo lv ing the same k i n d s of a toms. Such systems may be r e g a r d e d a s s p 2 h y b r i d i s e d to give 3 bonds a t 120°, and pd h y b r i d i s e d to g ive 2 co l inea r a x i a l bonds .

In o rder to ach ieve a system of minimum e n e r g y , mutua l r epu l s ion of l i g a n d g roups should l ead to a s i t u a t i o n in which each is a s e q u i d i s t a n t a s poss ib le from a l l the o t h e r s . In the case of 2 ,3 ,4 , & 6 coord ina t ion , a l l g roups can be e q u i d i s t a n t from the c e n t r a l P atom. This s i t u a t i o n i s imposs ib le in the case of 5 -coord ina t ion , bu t the two most symmetric a r r a n g e m e n t s a r e the t r i g o n a l b i p y r a m i d and the t e t r a g o n a l py ramid (Fig 1.7) . The ene rgy of a t r i g o n a l b i p y r a m i d a l conf igura t ion i s only s l i g h t l y l e s s ( * 1.5 kca l /mole) t h a n t h a t of the t e t r a g o n a l p y r a m i d , bu t i t a p p e a r s from theo re t i ca l c o n s i d e r a t i o n s and expe r imen ta l measurements t h a t the t r i g o n a l b i p y r a m i d a l a r r a n g e m e n t wil l u s u a l l y be p r e f e r r e d .

The t e t r a g o n a l p y r a m i d a l a r r a n g e m e n t is adop ted by an u n s t a b l e t r a n s i t i o n s t a t e d u r i n g r e a r r a n g e m e n t of the t r i g o n a l b i p y r a m i d

32 1,3 (Section 1.4). Stable tetragonal pyramidal molecules do exist in the solid state but examples are comparatively rare (Chapter 13.2).

In compounds where the phosphorus coordination number exceeds 6 and may be as high as 10, as in some metal phosphides (Chapter 2.2), the bonding may be only par t ia l ly covalent.

Known examples of the two, four or five-connected trivalent configurations depicted in Table 1.19 are comparatively ra re , but in a few instances stable compounds with the necessary number of added or subtracted elecrons can be isolated (13).

Me2N" *-NMe0 A1C1,

C l . C l -

Cl I

: P : I ci

Η N . C N 2 "

^ i N r ^ ^ C N (13) NC CN

Four and five-connected arrangements of this kind are more common with the heavier pnictide elements (14).

- y ^ :sbi :sb: (14)

Schemes of ð Bonding

( a )

ρπ - ρπ ρπ - απ ( b )

dir - dir ( c )

Figure 1 .12 Scheines of Π-Bonding

In forming multiple bonds phosphorus utilises either p or d Orbitals and may form bonds of the types :

P i r ( P ) — P i r ( X ) , d i r ( P ) — p i r ( X ) or dir(P)—dir(X),

where X is most commonly C,N,0 or S. Known compounds in which X is P,As,Sb,Se,Si,Ge,Sn or metal are smaller in number.

Phosphorus most often forms multiple bonds of the dif(P)—ñð(×) type and in this respect it resembles other second row elements such as Si or S (Fig 1 .12) . There is , however, continuing controversy over the importance (or existence) of ð -bonding in some phosphorus compounds and the discussion below should not be regarded with any degree of finality. d οο—p TT B o n d i n g

The degree of d orbital ð-bonding is determined by electron avai labi l i ty which is in turn controlled by the nature of the bonded atoms or groups. Highly electronegative substituent groups increase the effective positive charge on the phosphorus atom and thus favour the participation of d z2 and dx2_y2 orbitals in ð-bonding.

In many phosphorus compounds the bonded atoms have unshared electrons which are back-donated to fill the empty phosphorus d orbi tals . This probably occurs in the phosphoryl bond in POCl3 ,

1.3 33

where 3d^(P) — 2ñð(0) bonding takes place in addition to ó-bonding. Back-bonding of this kind frequently arises from 3áð(Ñ) — 2ñð (Í ) interactions in the case of phosphorus-nitrogen l inkages.

In tr ivalent phosphorus compounds the ð -bonding is usually weak but it is especially pronounced in tetrahedral compounds where it constitutes a significant addition to the basic ó bonding scheme. It may also occur in a few trigonal bipyramidal compounds.

( f )

Figure 1.13 Orbital Overlap Schemes for Ð Bonds (a) p z - d x z in POCl3> (b) ñ ÷- á ÷

2 in PF3, (c) p ^ d / in PO*", 2 2 3— (d) p w -d in PO, , (e) dat ive ir bond t r a n s i t i o n metal - phosphine, tf x - y 4

(f) dat ive ôô bond t rans i t ion metal - carbon monoxide.

Some typical te trahedral orbital schemes are shown in Fig 1.13. The symmetrically-bonded PO43" anion contains a double system of ð bonds equally distributed over all of the four l inkages, whereas in POCI3 the ð-bonding resides almost wholly in the phosphoryl l inkage. These cases correspond to the classical valence bond concept of resonance in the case of the P043" anion (15a) and a fixed 'double' bond in the case of POCl3 (15b).

0 0 II I

0—,Ρ—0 <—> o — P = O 1 I

� 0 — p — 0

I 0 = r P — 0

I 0

( a )

C l I

Cl— P = 0 I Cl

(b)

f2 S — P = S

NH„ (15)

( c )

The phosphoryl bond in symmetrical R3P=0 type compounds is exceptionally stable. This is usually attributed to the formation of two mutually perpendicular du—ñð type orbitals using two lone pairs on the oxygen atom. These overlap with two separate d orbitals of phosphorus, giving the symmetry, although not the strength, of a triple bond. In less symmetrical te trahedral molecules such as (15c), however, significant du—ñð interaction is possible with more than one substituent group, which compete in differing degrees for the ð -bonding. If the classical valence bond formula is to be used in such instances, the 'double' bond should be placed where the ð bonding is believed to predominate. * Se e addendum p 64.

34 1.3

Since the o r b i t a l ove r l ap occurs in the region n e a r e s t to the e lectron donor (Fig 1 .12b) , d u — ñ ð bonding is therefore p o l a r , bu t i t is weaker t h a n ñ ð — ñ ð b o n d i n g .

Since the d o r b i t a l s a r e be l ieved to be u n a v a i l a b l e in n i t r ogen , amine ox ides , R 3 N — 0 do not form double bonds and a r e marked ly less s t a b l e t han phosphory l compounds. Phosph in imines , R 3 P=NR\ and y l i d s , R3P=CR£ show ð-bonding l ike phosphory l R3P=0 compounds.

ñ ð — ñ ð Bonding Bonding of the 3ñôô(Ñ)-2ñð(×) t y p e , where X = C.N.O, i s be l ieved

to be p resen t in most ë 3ó 2 compounds. Cop lana r i t y is a p r e r e q u i s i t e for ñôô—ñð type bond ing , but

t h i s i s not n e c e s s a r y for άτ\—ñð bonding because of the geometric d i spos i t ion of the phosphorus d o r b i t a l s . This i s ref lec ted in the puckered r i n g systems found in most cyc lophosphazenes (Chapter 5) in con t r a s t to the f la t r i n g s in benzene and p h o s p h i n i n e .

o. Ö F 3 C - P = C F 2 [ I l + Y V _ p « C ( S i M e 3 ) 2 (16)

(a) V P ^ (b) ^ — \ (c) Of p a r t i c u l a r i n t e r e s t a r e -P=C compounds which, l ike those

con ta in ing -P=P- , can be s t a b i l s e d by the p resence of s t r ong ly e l e c t r o n - w i t h d r a w i n g g roups a s in F3C-P=CF2 (16a ) , or by e lect ron dé loca l i sa t ion a s in phosph in ine C5H5P (16b) , or by the in t roduc t ion of bu lky g roups such a s Bufc , t r i m e t h y l s i l y l , or 2 ,4 ,6 t r i - t e r b u t y l p h e n y l to sh ie ld the r e a c t i v e c e n t r e s ( 1 6 c ) .

The less s t a b l e -P=C compounds tend to d imer ise or form h igher polymers (17) .

R V R ' 2 R—P=CR'2 ^ R~"~PC / P ~ ~ R ( 1 7 )

R ^ R '

The same cons ide ra t i ons a p p l y to -P=N- compounds (18) (19)

*2N— P=NBu (Me Si)2N—P=NSiMe P r V — P==NBu (Me Si) N—-P=NSiMe (18)

/ N R ' 2 R—P=NR' * R—P^ ^>P—R ( 1 9 ^

Only a few s t a b l e compounds a r e known which con ta in a -P=P-l i n k a g e e . g . (20)

B u t ~~A / ~ Ñ â Ñ - Ë \ — B u t (MeQSi)QC—P«P—C(SiMeQ)0 (20) ΛΟ-. \ / ~3 ' 3 v 373

Less s t a b l e v a r i e t i e s will polymerise (21) . Phosphobenzene, PhP=PPh, the poss ib le ana logue of azobenzene , PhN=NPh, does not a p p e a r éï e x i s t .

1.3 35

2 R P = P R

R

R-p. J>-R (21)

Metal-Phosphorus and dir—ά-π Bonding 3p 3p 3p 2p

§ 8 § 0 (9QΤ ιW^> Ρ π - P i r Ρ π - Ρ τ τ d π - ρ π

Figure 1.14 Metal - Phosphorus Bonding Schemes d ï ï - d ð

In metal-phosphorus coordination compounds (Chapter 10) the principal bonding generally arises from the ó-donor capacity of the unshared electron pair on a tr i valent P atom. Back-donation from a filled metal d orbital to an empty phosphorus 3d orbital, áð(Ì)—3dir(P) , may also occur, but this is now believed to be generally weak. (Fig 1.14).

In the case of transition metal complexes such as Ni(PF3 )^ , the du—d ï ï bonding is probably significant and can be compared with the situation in the corresponding carbon monoxide complexes(Fig 1.13 f ).

( a )

P P

J|—>M M<r|~»M P P

( c ) (d )

B - -

M

(b)

P M«-J|->M

P

(22)

H& e ) (f ) (g)

In addition to ó bonding as in (22a) , bonding can arise from a sideways interaction of the ð orbitals formed between P2 units, with metal d orbitals (2 2b). Sideways bonding of this kind may stabilise otherwise unstable double-bonded P compounds, by withdrawing electrons. Situations (c-g) have now been established in various metallophosphorus compounds (Chapter 10).

Examples of some actual compounds are (23).

p

(OC) Fe4r-ll-*Fe<C0), p .

Me0Si 3 \

Me Si

P / ||->Ni

SiMe ?iMe3 P P̂

SiMe, SiMe,

(23)

The ð orbitals formed in ring systems can also interact as e.g. in such compounds as (24).

36 1.4

Mo

(24)

Similar sideways bonding arrangements occur with phosphaalkenes -P=C , phosphaalkynes, Ñ Î Ï , ð -bonded ring system s and -P=N compounds (25).

Bu C=pP

.Pt Ph3P

XPPh3

t „ ^ Bu'-cßW (25)

1.4 EQUILIBRIA AND STEREOCHEMISTRY

Optical Isomers When several different ligand groups are attached to a central P

atom, various isomers are possible. Pyramidal phosphines, Pabc, and tetrahedral compounds, Pabcd, can exist as mirror plane-related isomers which show optical activity (26). In the latter case the isomerism is analogous to that based on the asymmetric tetrahedral carbon atom, long established in carbon chemistry.

/?\ .'Vc I

c b a a ' V c (26)

Five and six-coordinated phosphorus compounds containing different l igands (Pabcde and Pabcdef) can show positional isomerism without necessarily involving optical activity (Chapter 13.2). Inversion and Pseudorotation

The interconversion of one isomer to another generally involves breaking of bonds, movement of l igands and re-forming the new isomer. In the case of pyramidal and trigonal bipyramidal compounds, however, intramolecular ligand exchange can also occur without bond breaking being involved. These latter processes are known as (pyramidal) inversion and (trigonal bipyramidal) pseudorotation respectively. Thus optically active pyramidal isomers can be interconverted by the process of inversion (27)(Chapter 13.2). Such isomers can often be isolated since phosphines are configurationally stable below about 100°C - unlike the corresponding amines which undergo rapid inversion at room temperature.

1.1 37

a b c ^ p v \ ^ ^ P - ^ (27)

a ^ b c

Trigonal bipyramidal isomers, whether optically active or not, are interconverted by the process known as pseudorotation. In some molecules this process may take place spontaneously, while in others it is inhibited.

Pseudorotation appears to be of two main types. The first type, known as Berry pseudorotation (BPR), involves the interchange of the two axial l igands with two of the equatorial l igands, the remaining equatorial arm functioning as the pivot. This operation proceeds via an intermediate tetragonal pyramid which is achieved with only a 15° distortion of the angles in the original trigonal bipyramid (28).

t > « _ 7/ ! > AN

J ^ C ^=± —/ τ=± — < a (28)

d d

The second type, known as Turnstile Rotation (TR), involves a rotation of a pair of arms (one axial and one equatorial) relative to the remaining trio of arms. Before this rotation takes place , a slight ini t ial distortion of about 9° is necessary in order that the ' p a i r ' and the ' t r io ' become symmetrically disposed with respect to the turnstile ax is . After the relative twist of the pair oand the trio in turnsti le fashion, a further angular adjustment of 9° takes place to restore the correct angular arrangement of the trigonal bipyramid (29).

?fl e — l V ^ e P ^ d (29)

1̂ ·* φ PS ÉË" The principles of pseudorotation have an important application

in the explanation of the reaction mechanisms of many phosphorus compounds (e .g . Chapter 13.2). Because of pseudorotation and pyramidal inversion possibilities, trigonal bipyramidal and pyramidal phosphorus compounds are said to be stereochemically non r igid. There is evidence that pseudorotation processes occur in arsenic compounds and a few other non-pnictide compounds such as Fe(CO)s .

Fluxional Molecules If the two (or more) al ternative configurations of a

stereochemically non-rigid molecule are chemically equivalent and have identical energies as e .g. in (30), the molecule is said to be fluxional.

Me P=CH—PMe F ; * FMe0P—CH=PMe_ (30) « J O O ό

Stereochemically non-rigid molecules such as PH3, which undergo

38 1.4

inversion, or PF5, which undergo pseudorotation, can be considered as simple examples of fluxional molecules. Tautomerism

If a molecule can exist in two (or more) alternative configurations which are not chemically equivalent, the process of interconversion is called tautomerism, and the two alternative configurations are known as tautomers.

Tautomerism is the result of two structurally dissimilar configurations being in rapid equilibrium so that at any instant both tautomers are present. This nearly always involves oxygen or nitrogen with a shift of a hydrogen atom and the position of a double bond within the molecule. The rates of interconversion of tautomeric forms vary widely, and if it is very slow, it may be possible to isolate both tautomers in a relatively pure state.

H 0 X k H 0 \ ^°

HO-P . Ñ æ (31) H<T H(T X H

( a ) ( b )

Some tri valent pyramidal molecxii.es; exist in tautomeric equilibrium with tetrahedral forms. Phosphorous acid e.g. may be written as (31). This compound exists in tetrahedral form in the solid state or in aqueous solution, although in many of its reactions it behaves as a trivalent molecule. Derivatives obtained by replacing the hydrogen by various atoms or groups R, can usually be isolated only in one form, depending on the nature of R. Tri-esters with 3 H atoms replaced, exist only in pyramidal form (Chapter 4.8).

Both tautomers often co-exist in solution, but the form containing the phosphoryl linkage usually predominates (32)-(34).

R \ k \ / H

HO—P " P x (32 )

„/ / \ R R 0

R 0 X OH R 0 V . 0

RO N N R ' !ÊÔ XNHR«

Κ 0 \Χ« , » \ ,* RO No RQX XOH

Equilibria of Trigonal Bipyramidal Molecules Many trigonal bipyramidal phosphorus molecules exist in

equilibrium with tetrahedral phosphonium cations and octahedral anions. In some instances all forms can be isolated and characterised. Phosphorus pentachloride e.g., exists as PCI5 in the vapour state, but as an assembly of PCU* PClö" in the solid state. Dimerisation also occurs to a small extent in some solvents (35)

1Ë 39

Cl Cl Cl ·^ '.^-Cl-^ l ^ - C l

c r ^ ^ c i ^ 1 } *^ci 2C 1" -ôÄ Cl' " - - v*1 «4U .m : p — c i

/ V Cl Cl Cl I Cl Cl Cl Cl Cl

In some cases trigonal bipyramidal and pyramidal molecules can exist in equilibrium. Thus in (36) the trigonal bipyramidal form exists in méthylène chloride solution, but in dimethyl furan the phosphite form predominates.

coco — ex: HOL

X o J i J <36> Resonance (Mesomerism)

Tautomeric (37) and fluxional (38) equilibria both involve atom transfer and a change in the site of the multiple bonding, but should not be confused with resonance (mesomerism) (39).

f? ? H ^ P f? , ? H {? Ph-y-N=P-Ph K ph-P-NH-P-Ph v Ph-P=N-P-Ph (37)

OH OH OH OH OH OH

Me P=N-PMe =CH_ HC=PMe -N=PMe_ (38) o Ä Ä 2i Ä o

Me3P=N-?Me « > Me f-N=PMe or &e P~N==PMe_| + (39)

Resonance generally involves two or more structures with identical or only slightly differing energies but no differences in the configurations of their nuclei. It is largely a theoretical concept with the true electronic structure lying somewhere between the alternative representations. There is no oscillation between these alternative representations, and the resonance structure is a hybrid which has a lower energy than any of the al ternative mesomeric s tructures. Extra resonance stabilisation is considered to result from this difference in energy.

O — O (40)

O O O O

0 — P = 0 < > 0 — P — 0 < > 0*=P—0 < > 0—P—0 t4JJ

L II I - l -0 0 0 0

While benzene is the most celebrated example in carbon chemistry (40), the orthophosphate anion (41) is probably the most commonly encountered resonance structure in phosphorus chemistry. One of the simplest examples in organophosphorus chemistry is phosphinine (42a) (8-176). Cyclohexaphosphene (42b) also appears to exist under certain conditions (10-176).

40 1.4

(a)

p - ^ p 11 I

(b)

(42)

Valence Bond Tautomerism Valence bond tautomerism involves equilibria between

configurationally similar structures which differ in the arrangement of their chemical bonding. It is related to the phenomenon of sigmatropic rearrangement encountered in carbon chemistry.

A spectacular example is provided by the P? " anion (43a), whose behaviour is analogous to that of bullvalene, CIQH 1 D (43b).

r/- Γ P>Tl (a)

Of / C H

I f II CHA—-CH

(43) (b)

In these 3-fold axially symmetric fluxional molecules, the P-P and C-C bonds are continually being broken and reformed between different pairs of atoms in such a way that the new structures all remain chemically identical.

(44)

By breaking one bond in the phosphorus anion, e .g. 1--2, 2—6 or 1—6 and forming 3—7, 3—5 or 5—7, the P atoms in the three membered ring are successively interchanged with the three bridge P atoms (44). Each of the seven P atoms can end up in any of the possible positions by appropriate rearrangements, leading to 7/3 = 1680 identical valence tautomeric forms. At room temperature this reversible dynamic process is rapid compared to the NMR time scale (Chapter 13).

Sigmatropic Re-arrangements - Phospha-Cope A sigmatropic rearrangement is defined as a migration of a ð

bond adjacent to one or more ó bonds, to a new position in a molecule, with the double bond ð system becoming reorganised in the process as e .g. in the diene (45).

Ph Ph

(45)

1Ë m

In the special case of the original compound being symmetrical the product is identical and an equilibrium is established. This is sometimes known as a degenerate Cope rearrangement (46)

(46)

Phospha-Cope rearrangements occur with some tetraphosphahexadienes (47).

R - P ^ ^ P - R R-P^C^P-R (47)

R-P^ ^P-R ^ R-P^ ^P-R R = Ph; R' = Me^iNPh R' R'

In the case of diphosphahexadienes, the two forms are not equivalent and such sigmatropic rearrangements appear to take place only in the direction which replaces ë3 ó2ñ with ë3 ó3 ñ (48).

T ?' R-P^ ^ÇH R-P--C^CH (48)

I > ! R-p^c^-CH2 R - P ^ ^ C I ^ R = Ph; R' = OSiMe3 4· à·

Reorganisation Reactions Intermolecular ligand exchanges occur with mixtures of some

tr i valent phosphorus compounds, and these are known variously as ' scrambling ' , ' reorganisation' or ' redistr ibut ion ' reactions. Such reactions must necessarily include bond breaking and re-forming in their mechanisms, whereas intramolecular ligand exchanges may not, as e .g . in the pseudorotation processes described above.

A mixture of PCI3 + PBr3 will spontaneously rearrange to produce a mixture of PCI3, PCl2Br, PClBr2 and PBr3. Any mixed trihalide or mixture of simple tr ihal ides can be made to approach equilibrium by a series of reversible reactions e .g. (49) (50). Reorganisations involving arsenic analogues are also known.

PF + PBr PF Br + PFBr (49)

«3 «i 2 2

PCI + P(NMe_), PCI NMe0 + PCl(NMe0)0 (50) o Δ o 2 £> 2 2

Reorganisation reactions have been observed with mixtures of tetrahedral compounds of phosphorus (51).

POCl + POBr POClBr + POC1 Br (51) 0 o 2 2

While some reorganisation reactions are spontaneous and immediate, others such as the interchange of different ester groups on tetrahedral phosphates are extremely slow and have high activation energies. Reorganisation reactions occur in polyphosphate melts (Chapter 3.3), and they also occur with pentacoordinated derivatives (Chapter 13.4).

42 1.5

1.5 TYPES OF REACTION

In general, phosphorus compounds prefer to react by electron-pair mechanisms, utilising the nucleophilic reactivity of the lone-pair electrons in the case of tr ivalent compounds, and the electrophilicity of the P atom in penta valent derivatives. However, some phosphorus reactions proceed by a free radical mechanism (Chapter 13).

Second-row elements are usually more nucleophilic than first-row elements of comparable basicity. This is often attributed to the relatively diffuse electron pairs on the larger atoms which are more polarisable (Table 1.20) and provide electrons more readi ly . In the case of phosphorus and nitrogen e .g . , the reactivity of Et3 P towards Mel (52) is greater than that of Et3N, although the latter is more basic.

E t 3 P Mel -> Et $Me i " (52)

The tr ivalent pyramidal arrangement of bonds generally represents the most reactive configuration of commonly encountered phosphorus compounds. In this arrangement the lone-pair electrons occupy what would otherwise be the fourth arm of a tetrahedral bond configuration. Such compounds may function both as nucleophilic (electron donating) (53) or electrophilic (electron accepting) (54) reagents.

Cl £—>BBr~ 3 3

Cl3P«—SMe3

(53)

(54)

This can be contrasted with trivalent nitrogen derivatives which, because of their lack of d orbital capacity, show only nucleophilic bahaviour (55)(56).

PClr 3H20 -> P(OH),

NC13 + 3H20 -> NH

t· 3HC1

3HOC1

(55)

(56)

Hydrolysis of phosphorus trichloride can proceed via an intermediate containing a decet of electrons whereas nitrogen trichloride cannot (57).

C I 3 P H20

C l

-> C I : P : O

Cl

-> C I : P : O H -HC1

Cl

_LH2°-- H C l e t c (57)

As nucleophilic reagents, tr ivalent phosphorus compounds can react rapidly with both electron-deficient centres (58) and electronically saturated carbon centres (59). Substitution at halogen is another common type of reaction (60). Even when phosphorus enjoys a full outer octet of electrons, it may accept more and show electrophilic behaviour (61) (62)

PC13 + 3CH3COOH

PPh„ + CH Br

-> 3CH COC1 -+

-> ph3£cH3 Br"

(H0)2PH0 (58)

(59)

1.5 43

PPh3 +

P (OEt ) 3

PCI3 +

Β Γ 2

+ ]

C 1 2

> Ph ?Br Br (60)

EtO.OEt > P(OEt) (61) 5

> PCI (62) 5

The phosphorus atom can show b i p h i l i c i t y a n d be both nuc leoph i l i c and e l ec t roph i l i c in the same r e a c t i o n , a s e . g . in the formation of phosphory l compounds ß*3Ñ=0, y l i d s R3 P=CR2 and phosph in imines R3P=NR. In these r e a c t i o n s the P atom i s nuc leophi l i c in forming ó - b o n d s , bu t a t the same time i t shows e l ec t roph i l i c b e h a v i o u r in accep t ing e lec t rons by back donat ion to form ð b o n d s .

Phosphorus forms s t r onge r bonds with oxygen t h a n do n i t rogen or a r s e n i c . The formation of the ve ry s t r ong phosphory l bond i s the d r i v i n g force for many r e a c t i o n s . Rear rangement r e a c t i o n s of phosphorus f r equen t ly involve the formation of t h i s l i n k a g e (63) (64) .

T? t

R - P - O R · > R - P ^ D , (63) vOR' Et

E t 0 ^ P ^ N ^ P ^ ° E t E tO^ / N \ ^ 0 E t O ^ , | -OEt 0 ^ , | ^ O E t ( 6 4 )

Í ^ ^ Í 7- E tN^ ^ N E t EtO^ *^OEt EtO*^ ^ 0

Some phosphorus compounds show d ienoph i l i c behav iou r and add to c a r b o n - c a r b o n or o ther mul t ip le bonds (65) (66) (67) .

+ RPC12 ± ß ^ C R1 c l ~ (65)

+ (RO)3P > ί^ X P ( 0 R ) 3 (66)

CH2=CH.CN + V^ï > R2P-CH2CH2CN W)

Examples of e l imina t ion r e a c t i o n s a r e (68) (69) .

^ C H 3 CH3 ^° Ψ2 C H 3 ' C H 2- P % C H 3 > }K + 1 (68)

^ 0 CH^ H CH2

Ph f.CH CH .Ph OH~ ^ Ph P + CH =CHPh (69)

Nucleophi l ic s u b s t i t u t i o n r e a c t i o n s u s u a l l y proceed r e a d i l y a t both t r i v a l e n t and p e n t a v a l e n t phosphorus c e n t r e s . An impor tan t except ion i s p rov ided by the phosph ine ox ides , R3P=0; these do not en ter in to s u b s t i t u t i o n r e a c t i o n s and show l i t t l e or none of the chemical r e a c t i v i t y a s s o c i a t e d with t h e i r ca rbon a n a l o g u e s the ke tones , R2C=0. Phosphonium compounds a r e p a r t i c u l a r l y s ens i t i ve to a t t a c k by nuc leoph i l i c r e a g e n t s , a l t hough they a r e s t a b l e to e lec t roph i l i c a t t a c k .

im 1.5

Me f.CH Ph OH" ^ Me PO + PhCH (70) 3 2 o «3

MeJi.CH Ph OH~ > Me0N + PhCH OH (71) o 2 o 2

The tribenzylphosphonium cation undergoes nucleophilic attack at the P atom (nucleophilic displacement at P) by the hydroxide anion to give eventually trimethylphosphine oxide (70). The corresponding ammonium cation is attacked by OH"* at the carbon atom and the products are quite different (71). (72)

C l ^ |

Cl

H O \ _ ^ N \ _ ^ O H O^^^NHx^^OH

6HC1 6HOH ^ HO>T fi-OH m o ^ f * ^ 0

HO XOH è' ^ O H

Substitution reactions which involve the production of phosphoryl linkages usually proceed readily. In (72) there is substitution at P by OH for Cl , followed by rearrangement. Nucleophilic substitution at P in pentavalent phosphorus halides can be effected by various nucleophiles (73)(74),

PC15 + PhNH2 > Cl P=NPh + 2HC1 (73)

R3P C 12 + R , ° H ^ R 3 P = = : 0 + R'Cl (74)

Phosphorus mechanisms have been much less studied than those involving reaction at carbon atoms. Many bimolecular substitution reactions in carbon chemistry proceed in one step, involving a trigonal bipyramidal transition state (SN2 reaction). The bonding of the entering group and the departure of the leaving group take place simultaneously and the trigonal bipyramidal configuration has only a transitory (<10~13 sees) existence (75a). Such reactions always lead to an inversion of configuration and if the groups R,R',R" are all different, the optical isomer will be obtained.

y . R \ / R

X: : > R—C—Y + Y—C--R (b) R / ^ R

R \ R—C—X R / o o R R

(75)

\ / -X / R

Y--C--X > Y—C—R (a)

I Ni R

Bimolecular substitution reactions occur at saturated tetrahedral phosphorus atoms in a similar way (76). The important difference in the case of P compounds is that the intermediate trigonal bipyramid has a finite existence ( % 10 ~13 sees), and it can be observed and sometimes isolated as a definite compound. In contrast to phosphorus, no trigonal bipyramidal carbon compound has ever been isolated or obtained with a lifetime sufficient for observation.

C 1N + - -HC1 f +/C1 ROH + P—Cl Cl > RO~P.--.Cl =� RO-P^-Cl Cl" (76)

Cl / / \ ^ π ci ci ci C1

1.5 45

The formation of a stable trigonal bipyramidal phosphorus compound is most likely when none of the substituents are good ' leaving groups' as e .g. in the formation of pentaphenylphosphorane (77). On the other hand elimination seems to occur most readily when a phosphoryl group is involved (78).

PhMgBr + Ph4PCl -> PhP o MgBrCl (77)

Ph„PI 4 NaOH -> HO—P—Ph ^ Ph PO + PhH + Nal (7Q) Ph' NPh

In the case of optically active tetrahedral carbon compounds the S N 2 reaction is stereospecific and always leads to an inversion of configuration. With phosphorus compounds, the corresponding reaction is not always stereospecific and does not necessarily lead to the inversion of configuration indicated in (75).

EdgeQ

Figure 1.15 Edge and Face Attack of a Tetrahedral Molecule

The ini t ial attack of a reagent can be regarded as taking place either on a 'face' or along an 'edge ' of the tetrahedron (Fig 1.15). Nucleophilic reagents tend to attack the face rather than the edge. If attack is on the face, it places the entering group in an apical position on the trigonal bipyramid which is formed by a small deformation of the existing bond angles. Attack on the edge of the tetrahedron places the entering group in an equatorial position. Elimination may then involve a group leaving from either an equatorial or an apical position, followed by relatively small deformations of the remaining bonds to give the tetrahedral arrangement again. There are thus four conceivable processes : (1) apical-apical elimination ( i . e . apical entering group followed another apical group eliminated, (2) apical-equatorial elimination, (3) equatorial-apical elimination, and (4) equatorial-equatorial elimination. Assuming the minimal possible distortions are involved in each case, processes (1) or (4) lead to inversion, while (2) or (3) lead to retention. Since the apical (axia l ) bonds on the trigonal bipyramid are usually the weakest, inversion arising from apical-apical elimination is normally expected to be the favoured process.

+ Y

\ ,

a c \ / b P Y

I x

- X

" p \ c (79) Y

46 1.5

a „ a c a b __ x a

/ + γ

\ / k \ / -x

X b P c >· b P Y ^ X P c > b P—-c

\χ i Y \τ (80) Although re ten t ion of conf igura t ion should be the r e s u l t of t ype

(2) (79) or type (3) p rocesses , r e ten t ion should a l so be the r e s u l t of an a p i c a l - a p i c a l type (1) p rocess if p seudoro ta t ion i s i nvo lved . Th is r e q u i r e s t h a t the in t e rmed ia t e t r i g o n a l b i p y r a m i d h a s suff ic ient lifetime

for pseudoro ta t ion to t a k e p l ace before a p i c a l e l imina t ion occurs (80) The other common mechanism for s u b s t i t u t i o n a t s a t u r a t e d c a r b o n , SN1 (75b) a l so h a s i t s ana logue in phosphorus chemis t ry . Moreover i t i s g e n e r a l l y be l ieved t h a t , in the case of both e lements , subs t i t u t i on r eac t i ons in t e rmed ia t e in mechanism between S N I and SN2 may sometimes t a k e p l a c e . In ca rbon chemis t ry the S N I mechanism involves an in t e rmed ia t e p l a n a r carbonium ion . Since the nuc leoph i l i c en t e r ing group may a t t a c k e i the r face of the p l a n a r carbonium ion with e q u a l p r o b a b i l i t y , a racemic mixture i s expected to be o b t a i n e d . In p r a c t i c e t h i s i s not a l w a y s ach ieved completely, because the nuc leophi le may have a t t a c k e d before the carbonium ion was p roduced .

Nucleophil ic s u b s t i t u t i o n r eac t i ons i nvo lv ing i n i t i a l l y p e n t a c o v a l e n t P atoms have no p a r a l l e l in ca rbon chemis t ry . There i s evidence t h a t they can proceed v ia t e t r a h e d r a l phosphonium ions or o c t a h e d r a l t r a n s i t i o n s t a t e s s ince e i t he r of these conf igu ra t ions can ex is t in equ i l ib r ium with t r i g o n a l b i p y r a m i d a l a r r a n g e m e n t s (81)(82) Knowledge of these r e a c t i o n s i s , however , a t p r e s e n t ve rv l imi ted .

Y" R4PX > R4P+ x" > R4PY + x" (81)

Rv ^ R Y" R. JR R X Λ R >>- X > Y _ ^ p C _ X > Y P R + X

R R R R (82) Some g e n e r a l r e ac t i ons of g r e a t impor tance in phosphorus

chemis t ry a r e a s follows : (1) Phosphory la t ion

P robab ly the most impor tan t r eac t ion i nvo lv ing s u b s t i t u t i o n a t the P atom is t h a t of phosphory la t i on (phosphory l t r a n s f e r r e a c t i o n ) , which e n t a i l s nuc leophi l i c d i sp lacement by nuc leophi le Y , on a phosphorus atom as in (83) , where X i s commonly OR,halogen, NR2 etc , and Y can be wa te r , a lcohol , amines etc

- \ \ Y + B—; P = 0 > B—P=0 + X (83)

X ã '

L iv ing o rgan i sms depend on r e a c t i o n s of t h i s type for ene rgy convers ion a n d pro te in s y n t h e s i s (Chapter 1 2 . 2 ) .

The term ' p h o s p h o r y l a t i o n ' i s used in o rganophosphorus chemis t ry to cover the t r a n s f e r of whole es te r i f i ed g roups such a s (RO)2PO- in a r eac t ion of t y p e ( 8 3 ) . I n reac t ion (84) , (PhO) 2 P(0 )Cl ( the e l ec t roph i l e ) i s s a i d to p h o s p h o r y l a t e EtNH2 ( the n u c l e o p h i l e ) .

(PhO)2POCl + EtNH2 > (PhO) P(0)NHEt + HCl (84)

1.5 47

If, however, the phosphorylating agent contains one or more free OH, e .g . (HO)2POCl, the non-esterified OH groups compete with the external nucleophiles and give byproducts.

Since most natural ly occurring compounds contain at least one free OH group, it is necessary to design phosphorylating agents which can transfer an unprotected group directly. Much work has been done in this area, and it has been found that reagents of the type

(HO)2P(0)X-Y-Z

are good phosphorylating agents if the electrons from the P-X bond can be accommodatedon Z. This requires the P-X bond to be weak and that Z should be strongly electron a t t rac t ing. If this be the case, A can be phosphorylated by a reaction of type (85).

H O N ^ 0 HO 0 P + HA > p ' + X=Y + HZ ( 8 5 )

HO X X - Y - Z HO X A

It is believed that the highly important phosphoryl transfer reactions (86) involving nucleophilic displacement on a P atom (the latter acting as an electrophile), can take place either by a pure SN1 or a Sis|2 mechanism or by a mechanism intermediate in type. The efficiency of a phosphate transfer by the more common two-step nucleophilic substitution reaction (86), depends on the reactivity of the nucleophile Y and on how good a ' leaving group' X i s .

X - — - P — - 0 >· X :

/ " X P 0

An important example of phosphorylation via a pentacovalent intermediate occurs in the enzyme-catalysed conversion of adenosine diphosphate to adenosine triphosphate (88).

o p o + M ++ o o o o o o Il H _ II H "9 II I! II / H II II II _

Ad-O-P-O-P-0 + 0 - P - 0 'H > A d - 0 - P - 0 - P - 0 - P - 0 ' >AdO-P-O-P-O-P-0 ( 88 ) I - I - I - I - I - I - \ H

+ - H O I - I - I -0 0 O n T n 0 0 0 2 0 0 0

s y n t h e t a s e

Y:

0 0

\ / P 1 0

\/° X p y

1 0

v V P . O nr Π �. � P Y

\ 0 0 / (86)

(Γ (87 )

ATP

(2) Ester Hydrolysis Hydrolysis reactions figure prominently in phosphorus chemistry

and are of part icular importance in bio systems. The hydrolysis of phosphate esters has received a great deal of fundamental study, but the mechanisms in many cases remain unsettled and only par t ia l ly understood .

All orthophosphate esters, in principle, are capable of hydrolysis according to scheme (89).

48 1.5

R \ + HOH R \ + HOH H ° \ + HOH H ° \ RO—P==0 > HO—P=0 > HO—P=0 > HO—P=0 (89)

RO - ROH RO - ROH R 0 - ROH HO

Diesters, which are of part icular importance in biochemistry, are strongly acidic (Table 3.21) and completely in the anionic form at physiological pH (90) They are , therefore fairly resistant to nucleophilic attack either by OH" or H20 (which makes the intervention of enzymes so important in biochemistry).

R 0\ ^° R 0 \ ^° + J^pf > > ^ + H+ (90)

R'O OH R'O 0

Three major processes have been considered in phosphate ester hydrolysis :

(a) A one-step nucleophilic SN2 reaction which implies inversion (91)

R'Ov - ^OR' O ^ P - O R V ^ E - O H H O - P ^ O (91)

(b) Nucleophilic attack involving an intermediate trigonal bipyramidal structure which rotates before elimination (92).

Cov o / o ï·^× o o

/ \ ^ I^OR HO-f — > ^ / \ 0 OR OH OR X) OH

(92)

(c) Intermediate formation of the planar P03~ anion, which is rapidly converted by H20 to H2P04~ (93).

0 0 S - ^ Ï +HOH Jj

RO—P—0 > RO" + 0"—P^" > HO—P—θ" (93) \ 0 - X 0 -ROH \Q-

Observed rates of hydrolysis vary enormously depending upon the structure of the ester and the experimental conditions used (Chapter 3.5) In addition, the intervention of· enzymes in bio systems can increase hydrolysis rates by as much as 106 . (3) Thermal Condensation

0 O t h e r m a l . . 0 0 M H condensa t i o n il .|

0~—P—OH + H O — P — θ " > o — P — O — P — O " + H 0 ( 9 4 ) I - + I - + * I - | _ +

2

0 (Na ) 0 (Na ) h y d r o l y s i s o 0 (Na ) z 2 4

Thermal condensation reactions which produce polyphosphates are of great industr ial importance. These involve the heating of solid acid phosphate sal ts whereby P-O-P linkages are formed with the elimination of water (94). Numerous long-chain, ring and cage compounds can be produced by reactions of this kind (Chapter 3.3).

The reverse of thermal condensation, which involves the spli t t ing of P-O-P linkages, is liable to occur with any condensed phosphate when in aqueous solution. This hydrolysis reaction is also of great importance in phosphorus chemistry.

1.5 49

(4) Oxidat ion

Bu3P H 2 ° 2 . ^ B u 3 P = o (95)

(PhO) P °3 >- (PhO) P = 0 (96) J 3

T r i v a l e n t ë3 ó 3 phosphorus compounds a r e r e a d i l y oxid ised by oxygen, ozone, hydrogen perox ide and other o x i d a n t s to g ive the more s t ab l e ë 5ó 4 d e r i v a t i v e s . Such r e a c t i o n s a r e of i n d u s t r i a l impor tance . In these often v igorous r e a c t i o n s , the formation of the s t a b l e P=0 bond i s cons idered to be the d r i v i n g force . The ox ida t ion of PCI3 to POCI3 h a s been c h a r a c t e r i s e d a s a r a d i c a l - c h a i n p rocess with the Cl atoms a s the p r i n c i p a l cha in c a r r i e r s (97)

°2 - P C 1 3 PC13 + Cl > PC14 ^ PC14°2 ^ * 2P0C1 + Cl (97)

(5) Reduction Powerful r e d u c i n g a g e n t s such a s L1AIH4 a r e needed for t h i s

type of r eac t ion (98)- (100) .S i l icon compounds such a s S1HCI3 or PhSiH3 may a l so be used in some c a s e s .

R P(0)OH i l Ü l H 4 — > R PH (98)

2 l a " ^ - » - ,

• 3 - — ^ 3 - ^ V

RPClo i l i^lH4 * RPH0 (99)

R PO 5 ^ 3 _ ^ ( 1 0 0 )

(6) Michael i s -Arbusov Reaction (Arbusov React ion)

(RO)3P + R'X > (RO)2P(0)R' + (RO)X (101)

One of the best known and most impor tan t r eac t i ons in o rganophosphorus chemis t ry is the p roduc t ion of P-C l i n k a g e s by the Arbusov r e a c t i o n . O r i g i n a l l y formulated a s a r eac t ion between a t r i a l k y l phosph i t e and an a l k y l h a l i d e (101), t h i s r eac t ion was l a t e r found to be more g e n e r a l and can be wr i t t en a s (102), where A,B can be a l k y l , a r y l , p r i m a r y or s econda ry a l k o x y , a r y l o x y , d i a l k y l a m i n o e t c .

^ A A Ñò-Â + R'X > R' — P—-B + RX (102)

X 0 R ^ 0

The mechanism of t h i s r eac t ion invo lves a q u a s i phosphonium in te rmedia te which in some cases can be i so la t ed e . g . when A & B = PhO.R =Ph, R'X = Mel (103).

(PhO) P(OPh) + Mel ^ (PhO)oPMe i " ^ (Ph0)oPMe + Phi (103) £ 2 21

3Ph (7) Wittig Reaction

R P=CH + R C = 0 > RQP=0 + H C=CR0 (104) ό et £i o 2 2

50 1.5

The Wittig Reaction (104) represents another highlight in organophosphorus chemistry. It has important industr ial application and has been successfully used in the synthesis of alkenes and natural products.

In this reaction, a ketone or aldehyde reacts* to give init ial ly two isomeric betaines (105).The relative rates of formation and decomposition of the diastereoisomeric betaines control the stereochemistry of the olefin mixture eventually produced. It is known that many factors can influence the stereochemical outcome of the Wittig reaction. Although this reaction frequently leads to mixtures of stereoisomers, by suitable choice of reagents and reaction conditions, the reaction can be made stereoselective.

In cases where betaine formation is reversible, the thermodynamically more stable isomer will be formed before elimination occurs. This is normally (105b), which leads to the t rans alkene. Generally trialkylphosphonium ylids, or those containing stabilising groups, give mainly t rans alkenes.

^

R" ( a )

R 3 p N O — C-H

R"

R - c - H

R„-c^H

R ?-CHR' 0=CHR"

R 3 f ^c r" H

°--v" betaine

R 3 ? — C C

( b )

31 ,-H 0 Q-R" \

H

(105)

R - c - H

> II

trans oxaphosphetan

The amount of eis isomer can be increased by the use of protoic solvents, which presumably solvate (b) reducing the interaction between P and O and allowing (105a) to be formed. In some cases the intermediate betaine (106) can be trapped by protonation or complex formation with lithium salts (107), or the oxaphosphetanes may be isolated or detected in solution (108).

Different mechanisms have been proposed for the Wittig reaction.

R3P=CR'R"

0=CR"»R"

R 0 £—CR'R" 3_ I

O—CR"' R"

R P—CR? R" 3 I ! '-

O—eR'"R"

R P + CR' R"

I II ( 1 0 6 ) O CR"'R""

Ph P=CHR + PhCHO - * Ph fcHR — H I -^Ph3PCHRCH(OH)Ph I _ 3 /

O—CHPh L i B r - > P h fcHRCHPhOLi Br"

(107)

Ph P=C=PPh «3 o

(CF3)2CO ■^ Ph3P—f=PPh3 O — C ( C F 3 ) 2

(108)

1.6 51

1.6 BOND STRENGTHS & BOND LENGTHS

The d i f f icu l t ies in o b t a i n i n g r e l e v a n t v a l u e s for bond s t r e n g t h s or the c losely r e l a t e d bond ene rg i e s in chemical compounds a r e well known. Various methods have been used for t he i r computa t ion , but in the a r e a of phosphorus chemis t ry the topic h a s been much neg lec ted . The o v e r a l l p i c tu r e r emains ske tchy and s t r i c t l y comparab le va lues a r e a v a i l a b l e only for l imited g roups of compounds.

Tab les 1.23 & 1.24 con ta in some of the d a t a ob ta ined but the va lues given a r e bes t r e g a r d e d a s t e n t a t i v e . I t should be remembered t h a t the s t r e n g t h of a g iven bond may v a r y somewhat from one compound to ano the r , due to in f luences of n e i g h b o u r i n g bonds . The va lues l i s t ed refer to the thermochemical bond e n e r g i e s , E. These a r e the q u a n t i t i e s which, when summed over a l l the bonds p r e s e n t , g ive the hea t of formation of the molecules from atoms, r e fe r r ed to 298 K.

TABLE 1-23 Homopolar Bond Energies i;ies

C

88

62

Si

74

28

Ge

39

(k cal mol )

N

64

69

P

61

34

As

34

0

50

84

S

66

30

Se

44

F

37

Cl

57

Br

47

TABLE 1-

Heteropola

P-H 77

P-F 126

P-Cl 79

P-Br 63

P-I 44

P-C 65

P-N 70

P-0 86

P=0 130

•24

r Bond Energies (k

N-H 93

N-F 65

N-Cl 46

N-C 73

N-0 50

N=0 145

As-H 59

As-F 116

As-Cl 69

As-Br 58

As-I 43

As-C 48

cal mol )

Sb-H 70

Sb-F 108

Sb-Cl 74

Sb-C 47

C-H 99

C-F 105

C-Cl 78

C-N 73

C-0 85

C=0 174

Si-H 70

Si-F 135

Si-Ci 86

Si-N 77

Si-0 88

S-H 81

S-Cl 60

52 1.6

An important point to note with regard to the data in Table 1.23, is that the ð bonds formed between second row elements are proportionally weaker than those formed with first row elements. This is usually attributed to the reduced orbital overlap resulting from a larger size difference between the s and p orbitals in the case of the second row elements (Section 1.3).The ð bonds formed by phosphorus are generally weaker than those formed by carbon, but stronger than those formed by silicon.

The bond dissociation energy, D, used on occasion, represents the energy required to break the bond, referred to 0 K. The two quantit ies, E and D, are comparable only in diatomic molecules where D relates directly to the heat of dissociation and E to the heat of formation. In polyatomic molecules the value of D for a given bond may include configurational changes consequent upon breaking i t . Values of D relating to multiply-bonded diatomic molecules are listed in Table 1.25 and some further comparisons are made in Table 1.26.

TABLE 1-25

Dissociation Energies (Dn) for Multiply-Bonded Diatomic Molecules

PN 164 k cal mol"1 N2

P2

As2

sb2

Bi2

226

117

91

71

40

NO

PO

AsO

SbO

BiO

150

140

113

102

85

NC

PC

194

159

TABLE 1-2 6

Comparison of Bond Energies for Carbon, Nitrogen & Phosphorus k cal mol"

P—P 61 P—N 55 N—N 64 P—C 65 C—C 88

P=P 95 P.ssN 110 N=N 133 P=C 110 C=C 150

P=P 117 P=N 164 N=N 226 P~C 159 C^C 200

The stabili ty of a given bond can be very dependent upon the treatment to which it is subjected, and a high bond strength does not necessarily guarantee high stabili ty of the compound under all conditions. Some of the figures given in the above tables are only tentative, but in general it is found that :

(1) P—H bonds are weaker than N—H or C—H, but stronger than As-H and are comparable with Si—H or S—H.

(2) P—X bonds (X= halogen) are roughly as strong as C—X and stronger than N—X or As—X.

1.6 53

(3) P—0 bonds are stronger than N—0 or C—0.

(4) P—P bonds are somewhat weaker than C—C , but comparable with N—N, Si—Si or S—S and stronger than As—As.

(5) P—C bonds are a l i t t le weaker than C—C or N—C, but stronger than As—C.

(6) P—N bonds are fairly strong and comparable with P—P. They are a little stronger than N—N or P—C.

(7) P=P bonds are considerably weaker than N=N or C=C and a l i t t le weaker than P=N or P=C.

(8) P=C bonds"] comparable with each other. A little stronger P=N bondsj than P =P but weaker than C=C or N=N.

(9) P=0 bonds are much stronger than P—0 and stronger than P=S, but somewhat weaker than C=0 or N=0.

(10) P=S bonds are weaker than P = 0

(11) P=P bonds are weaker than N=N, P=N, P=C or C=C, but stronger than As=As

(12) P=N bonds are probably the strongest formed by phosphorus.

(13) P=C bonds are a li t t le weaker than P=N but stronger than

P s P . The P—P bond is very resistant to oxidation and hydrolysis, and in this respect it is more stable than C—C. Corresponding linkages to other Group IV elements are generally less stable:

P—C > P—Si > P—Ge > P—Sn > P—Pb Pnictide inter-element single bonds become weaker as the

pnictide atomic weight increases- the same probably holds for inter-element multiple bonds P=E and P=E :

P—P > As—As > Sb—Sb

P—P > P— As > P—Sb

The great strength and stabil i ty of the phosphoryl P==0 linkage dominates a considerable part of phosphorus chemistry, but when the 0 atom is involved in a bridge as e .g. in P—0—P or P—0—C linkages, this generally results in hydrolytic instabi l i ty . The P—0—P linkage is , however, more resistant to hydrolysis than P—0—As, As—0—As, P—0—S or S—0—S.

The P—0—C linkage is more stable than the analogous linkages with other Group IV elements i . e . :

P—0—C > P—0—Si > P—0—Ge > P—0—Sn.

Compounds containing P=S or P—S linkages tend to be less stable, both thermally and hydrolytically, than their oxygen

54 1.6

a n a l o g u e s . Selenium compounds a r e u s u a l l y l ess s t a b l e t h a n t he i r th io ana logues :

P = 0 > P = S > P = S e

Like the P - - 0 bond, the P - -N bond shows v a r y i n g degrees of ð -bond ing in different compounds, and these bonds a r e a s soc i a t ed with a r a n g e of s t r e n g t h s and s t a b i l i t i e s . In many compounds the P—N bond i s c o n s i d e r a b l y more s t a b l e under a l k a l i n e t han under ac id cond i t ions . Bond Lengths

A va lue for the ó or ' s i n g l e ' bond l eng th between two atoms can be ob ta ined by simple add i t ion of t he i r s ing le bond cova len t r a d i i . Such covalent r a d i i a r e o b t a i n a b l e from exper imen ta l de te rmina t ions of in te ra tomic d i s t a n c e s in the re spec t ive e lements . In p r a c t i c e , expe r imen ta l ly determined d i s t a n c e s between u n l i k e atoms often differ from the va lue s computed from covalen t r a d i i , and the l eng th of a g iven P-X bond will v a r y to some degree between one compound and a n o t h e r , a s e . g with P-F and P-N (Table 1.27). On the other h a n d , with some s ing l e bonds such a s P-P and P-C these v a r i a t i o n s a r e g e n e r a l l y l e s s marked .

TABLE 1-2 7 Varia t ion of P—F and P—N (Single) Bond Lengths __

HPF..6H 0

Me2N.PF2

MeO.PF

P0(0PF2)3

KP02F2

(PNF2)3

Covalent radius

Covalent radius (correc

1.73

1.61

1.59

1.58

1.57

1.51

sum

sum ted)

1

1

k

.82

.65

NaHPO NH

(Me2N)3P

Me2N.PCl2

(NH2)3P.BH3

Me2N.PF2.B4H8

Ph3P.S.PPh3

1.

1,

1,

1,

1,

1,

.77 A

.70

.69

.65

.59

.57

1.84

r.76

The exper imen ta l va lue is u s u a l l y shor tened from the cova len t r a d i u s sum, and t h i s may a r i s e from ( a ) p a r t i a l ionic c h a r a c t e r of the bond, or (b) the occurence of ð bond ing . Unless the bond i s homopolar, i t will have some ionic c h a r a c t e r , the amount of which i s r e l a t ed to the e l e c t r o n e g a t i v i t y difference between the two atoms concerned . The Schomaker-Stevenson empi r ica l correc t ion (109) enab le s t h i s effect to be c a l c u l a t e d .

r * > r « = covalent radii A B ΓΑΟ = r * + r o - 0 . 0 9 ( x - x ) (109)

¢á A Φ A B x x = e l ec tronegat iv i t i e s Any bond sho r t en ing beyond t h a t expected from e l ec t ronega t i v i t y

difference i s p r o b a b l y a t t r i b u t a b l e to ð c h a r a c t e r of the bond . Assuming t h i s to be so, d a t a such a s given in Table 1.28 can be ca l cu l a t ed for the amount of ð c h a r a c t e r in v a r i o u s types of bond.

1.6

TABLE 1-2 8

Calculated n~ Bond Orders

p?

P 4 PH?

P F3 PCIS

PMe3

F,PO

Bond

P-P

P-P

P-H

P-F

P-Cl

P-C

P-0

P-F

Length

1.890

2.205

1.424

1.546

2.000

1.87

1.56

1.52

ð-ÂÏ

2.0

0

0.1

0.2

0

0.1

0.4

0.3

CI3PO

F3PS

C13PS

P F5

Bond

P-0

P-Cl

P-S

P-F

P-S

P-Cl

P-F

P-F

eq

ax

Length

1*45

1.99

1.85

1.51

1.94

2.01

1.57

1.59

ÔÃ-ÂÏ

1.0

0

1.0

0.3

0.4

0

0.2

0.1

3-

0.2 0.4 0.6 Τ.8

π - bond order

1.0

The tetrahedral symmetry of PO

allows the formation of two strong ^

bonding orbitals with 3d 2 2 and

3d 2 of P with 2p Ttanâ * ~y 2pTrof

each 0 atom. In this valence bond

language which allows a total -rrbond

order of 2, each P-0 bond has a η-bond

order of l+l - } . Point A corresponds

the P-0 distance of 1.54 A in the PO ~

ion with bond order of i, while point

B corresponds to the single bond dist-

ance of 1.71 A with bond order of 0.

In other tetrahedral environments the

IT bonding is as follows :

RO^O.4 0.2Ό ^Ρ-Ο^ 1.0

RO-P-0 ->-0*-PM)~ ^Ρ-Ο-ΡβΟ

R0/0.8 θΤβ 0" ^P-0^0.33

Figure l.l6 n-Bond Order - Bond Length Relationship

In the case of P — 0 bonds there is a s t raight line relationship between bond length and ð -bond order (Fig 1.16). As a result ïßð -bond shortening, the phosphoryl bond (e .g . in POCI3 ) is always found to be significantly shorter than other phosphorus-oxygen linkages of the type P-0- (e .g . in P4O10). A similar difference is found on comparing the thiophosphoryl P=S distance with P-S- (Table 1.29).

Apart from bond shortening, evidence for ð bonding rests on various other ^factors. The bond energy of P=0 is very much greater than that of N—07 thus implying extra bonding in the case of the phosphorus compound. The bond strengths of P--0, P--C and P--N groupings are generally greater than those of analogous groupings where P is replaced by N (Table 1.26).

Infra red stretching frequencies move to higher values when

56 1.6

multiple bonding is present. This is evident on comparing v (P=0) and v(p=N) with v(P—0) and v (P—N) (Chapter 14.2)

On the basis of electronegativity difference, the dipole moments of R3PO compounds should be greater than those of corresponding R3NO compounds. The reverse is in fact the case, and this presumably arises from the back donation of electrons in 3áð(Ñ)--2ñð(0) bonding which reduces the effective dipole i .e Ñ*.ºÔ-0.

The stabil i ty of a phosphoryl bond is generally related to the electronegativities of the remaining substituents on the P atom. Highly electronegative groups tend to increase the positive charge on the P atom and thus increase the bond strength. The infra red stretching frequency, v (P=0) increases directly with the sum of the electronegativities of the substituent groups, thus indicating increasing bond strengths (Chapter 14.2). The great strength of the phosphoryl linkage is indicated by the almost universal preference for the phosphonate form when alternative tautomeric forms are possible. Electron délocalisation and the equivalence of bonds in ring systems can only be explained satisfactorily by assuming IT- bonding is taking place (Chapter 5.7)(Chapter 8.5).

It has already been pointed out that in the case of phosphorus the magnitude of the ð -bonding energy is much less in relation to the ó-bonding energy than is the case with nitrogen (Table 1.23). The extent of ð bonding is often uncertain and there has been much argument in the case of some individual compounds. Since in tetrahedral compounds the ð -bonding is not necessarily confined to the formal 'double' bond, the remaining ' s ingle ' bonds may be shorter than they are in pyramidal compounds where ð -bonding is generally believed to be absent. However, spectroscopic evidence for restricted bond rotation, suggests that in a few instances some ð -bonding may be present in pyramidal and trigonal bipyramidal phosphorus compounds (Chapter 13.2).

Crystal structure measurements on several thousand compounds, together with electron diffraction and microwave data from a smaller number of relatively simple gaseous molecules, indicate that in most compounds the bond lengths lie to within about + .05 A of the selected characteristic values given in Table 1.29. Larger variations outside these limits can occur and in these cases ð bonding or other effects may operate as e .g. in metal-phosphorus bonding (Table 10.5 ) .

Factors known to influence bond length include the valency state of the P atom (ë) , its coordination scheme (ó), the electronegativities of the remaining substituents, the interaction of surrounding molecules and effects of crystal s tructure.

Although the orbitals employed in á ð-ñð bonding are different from those utilised in ñ ð -ñð bonding, this is not necessarily reflected in any significant difference in bond lengths between similar pairs of atoms:

TABLE I-29

Representative Bond Lengths for Phosphorus Compounds (A)

Bond Length Compound Bond Length

P-H 1.44 PH P-Se 2.24

P-F 1.57 PF3 P=Se 1.96

Compound

P„Se0

Et3PSe

1.6 57

Table 1-29 continued

P-Cl

P-Br

P-I

P-B

P=B

P-C

P=C

P=C

P-N

P=N

PSN

P-0

P-0~

P=0

P-S

p-s"

P=S

2.04

2.22

2.52

1.96

1.83

1.85

1.66

1.54

1.77

1.57

1.49

1.64

1.54

1.45

2.13

2.03

1.88

PC13

PBr3

PI3 PB

(mes)pPB(mes)

PMe3

Ph3P=CH2

P^CH

NaHPO NH

Ph-P=NCfiH Br 3 6 4 P̂ N

P / . ° * 4 6 LiMnPO, 4 POCI3

(F2P)2S

Et2PS2Na.2H20

PSCI3

P-Te

P=Te

P-P

P=P

P=P

P-As

P-Sb

P-Bi

P-Al

P-Ga

P-In

P-Si

P-Ge

P-Sn

P-Pb

P-Be

P-Mg

2.50

2.37

2.22

2.03

1.87

2.35

2.57

2.79

2.53

2.44

2.69

2.26

2.31

2.53

2.78

2.08

2.59

(Bu3PTe)2Te

Bu3PTe

P2H4 (Bu3C6H2P)2

P2 (Bu3C6H2PAs)2

(Bu3C6H2PSb)2

Bi2Br6(PMe3)4

Me3Al-PMe3

(Bu2Ga( PH2))3

(Me2InPBu2)2

P(SiMe3)3

P(GeH3)3

P7(SnPh3)3

(Pb( PBu2)PBu2)2

C^Me^BePBu^ 5 b 2

Mg(PHPh)2

Observed bond l e n g t h s in 5 and 6-coord ina ted phosphorus compounds a r e g e n e r a l l y g r e a t e r t h a n in t e t r a h e d r a l compounds. In t r i g o n a l b i p y r a m i d a l compounds somewhat longer l e n g t h s and lower s t r e t c h i n g f requenc ies ( \>) of the a p i c a l compared to the e q u a t o r i a l bonds , i n d i c a t e g r e a t e r s t r e n g t h s of the l a t t e r . These poin ts a r e i l l u s t r a t e d by the d a t a in Table 1.30.

TABLE 1-3 0 Changes of Bond Length with Coordinat ion.

PCI . P0C1„ PCI, PCl r PCI,

P__ c i (A)

P—0

2.04

P ( O E t ) ,

1 .60

1.98

1.54

1.90 2 . 1 2 ax 2 . 0 2 eq

P(OPh) c

1 .66 ax 1.60 eq

2 . 1 4

φο]; 1.71

In common with Al—0, Si—0 and As—0 bonds , the a v e r a g e P—0 d i s t ance in o c t a h e d r a l coord ina t ion is about 10% g r e a t e r t han the value in t e t r a h e d r a l coord ina t ion (Table 1.31).

58 1.6

TABLE 1 - 3 1

Comparison of Tetrahedral and Octahedral Bond Distances (A)

Four-coordination

Six-coordination

Covalent (tetrahedral) radius sum

Al—0

1.74

1.91

1.99

Si—0

1.62

1.78

1.91

P—0

1.54

1.69

1.84

As—0

1.68

1.83

1.94

The a v e r a g e va lue s for ionised P—0" l i n k a g e s in t e t r a h e d r a l compounds depend on the to t a l n e g a t i v e c h a r g e and the number of 0 atoms which a r e expected to s h a r e the a v a i l a b l e ð bond :

o P—0 (A) O/P/O

0^p^o phosphate 1.535 + 0.020 103-115

-p===0 phosphonate 1.51 .03 110-115 0" °~"

^ P \ phosphinate 1.49 .03 113-123 "0

-P=0 phosphoryl 1.46 .05

Some ove r l ap of these r a n g e s is found because other fac tors a l so inf luence the f ina l bond leng th adopted in a n y i n d i v i d u a l compound.

Of spec ia l i n t e r e s t in b iochemis t ry a r e P—0—P and P—0—C groups which a r e found to have a v e r a g e dimensions :

p"CT -^*» p P ^ ^ - ^ ^ C ll lUJ 1 2 0 - 1 8 0 * 1 1 5 - 1 3 0 °

The P/O/C ang l e g e n e r a l l y l i es wi thin a smal le r r a n g e t h a n P /O/P , except in r i n g compounds where i t may be a s low a s 105° .

By t a k i n g the p robab l e to t a l bond o r d e r s toge ther with the exper imenta l bond l eng ths in Table 1.29, bond l eng th vs to t a l bond order cu rves can be cons t ruc ted (Fig 1.17). Such cu rves might be used to es t imate bond order from measured bond l e n g t h s .

Ex i s t ing c r y s t a l s t r u c t u r e and spec t roscopic d a t a i n d i c a t e the c h a r a c t e r i s t i c r a d i i g iven in Table 1.32. can be a s soc i a t ed with p h o s p h o r u s .

TABLE 1-3 2

Characteristic Covalent Radii for Pnictide Elements (A)

Single bond

Double bond

Triple bond

N

0.74

0.61

0.50

P

1.11

1.01

0.94

As

1.21

1.11

Sb

1.41

1.39

Bi

1.46

1.7 59

+ 1.80

NaHPO NH

1.60

PsCH

P = N

+ 1.40

2.0

Total Bond O r d e r

Figure 1.17 Total Bond Order - Bond Length Relationships

1.7 PRACTICAL CLASSIFICATION OF PHOSPHORUS COMPOUNDS

Until about 40 y e a r s ago almost the whole of phosphorus chemist ry could be d iv ided between t r i v a l e n t ( p y r a m i d a l ) ë3 ó3

compounds and p e n t a v a l e n t ( t e t r a h e d r a l ) ë5 ó 4 compounds (Section 1.2). I n o r g a n i c phosphorus chemis t ry dominated the field and the extent of known o rganophosphorus chemis t ry was s t i l l very l imi ted .

Since t h a t t ime, many more compounds i n c l u d i n g those with a l t e r n a t i v e combinat ions of va lency s t a t e s and coord ina t ion schemes have been d i scove red . Although i n o r g a n i c phosphorus compounds remain by far the most impor tan t commercial ly , the chemis t ry of o rganophosphorus compounds h a s evolved r a p i d l y and now r e p r e s e n t s a s i zeab le and exp los ive ly e x p a n d i n g p a r t of the whole.

As s t a t ed in the in t roduc t ion (Section 1.1), i t i s a t the p r e sen t time convenient to recognise four major c l a s s e s of P compounds :

60 1.7

Η Ο / ^ Ο

phosphenous acid

( a )

phosphenic acid

(b)

.OH P—OH

^ΟΗ

phosphorous acid

(c)

0 II

i » v HO \)H OH phosphoric

acid

(d)

OH

1* HO OH OH

tetrahydroxy phosphonium

(e)

OH 1 ^OH H ° - fC 0 H OH

pentahydroxy phosphorane

(D

(1) Oxyphosphorus compounds, which con ta in cova len t P—0 l i n k a g e s , (2) Organophosphorus ( ca rbophosphorus ) compounds which conta in

P--C l i n k a g e s . (3) Azaphosphorus compounds which conta in P--N l i n k a g e s (4) Meta l lophosphorus compounds which con ta in P—metal l i n k a g e s .

Some compounds will i n e v i t a b l y belong s imul t aneous ly to more t h a n one of the above g roups and the i r c l a s s i f i c a t i on will be somewhat a r b i t r a r y . In these cases the dominat ing or most i n t e r e s t i n g fea tu re should determine into which major c l a s s the compound i s p l aced .

There remain r e l a t i v e l y few P compounds which a r e devoid of any of the l i n k a g e s c h a r a c t e r i s i n g these major c l a s s e s . There a r e , however, the impor tan t phosphorus s u l p h i d e s (Chapter 2.4) and the phosphorus h a l i d e s (Chapter 2 . 6 ) .

Phosphorus chemis t ry is dominated by compounds with P—0 l i n k a g e s . Most of these a r e de r ived from the p a r e n t compounds (111), of which only o r thophosphor ic a c id , the tau tomer ic form of phosphorous ac id and poss ib ly the t e t r ahydroxyphosphon ium ca t ion , have any r e a l e x i s t e n c e . _ _.. Ë „ 0Ç

HO^':>-OH HO-"|*^OH

OH ( H I ) hexahydroxy

phosphoride

(9)

Der iva t ives of a l l (111) a r e known, bu t wi thin t h i s oxyphosphorus group the ë5 ó4 i n o r g a n i c phospha t e s (from H i d ) remain by far the most numerous and impor tan t (Chap te r s 3 & 6 ) .

Organic phospha te e s t e r s (Chapter 3 .5 ) , based on P-O-C l i n k a g e s a re a lso oxyphosphorus compounds. Phosphorus b iochemis t ry is almost exc lus ive ly concerned with such phospha te e s t e r s (Chap te r s 11 & 12) .

Within the o rganophosphorus group i t i s often convenient to inc lude what a r e s t r i c t l y o r g a n i c phosphorus compounds ( i . e . an o rgan ic group p r e sen t but no d i r ec t P—C l i n k a g e ) . Organophosph i t e s , o rganophosphoranes (Chap te r s 4 & 8)and sometimes o r g a n i c phospha t e e s t e r s (Chapter 3.5) a r e examples of t h i s .

Azaphosphorus compounds will be cons idered to inc lude any d e r i v a t i v e with a P--N l i n k a g e , w h e t h e r o r g a n i c or i n o r g a n i c ( C h a p t e r 5 ) .

Meta l lophosphorus compounds a r e ba sed on P—Metal l i n k a g e s , but i t is sometimes convenient to inc lude compounds c o n t a i n i n g P and metal atoms which a r e devoid of P--Metal l i n k a g e s . A major d i f fe ren t ia t ion can be made between i n o r g a n i c metal phosph ides (Chapter 2.2) and compounds in which the P atom is l inked to ano the r k ind of atom in add i t ion to a meta l . (Chapter 10) .

I t will sometimes be useful to c lass i fy phosphorus compounds in accordance with the presence of two c h a r a c t e r i s t i c bonds e . g .

C—P—0 o rgano -oxyphosphorus compound N—P—0 a z a - o x y p h o s p h o r u s compound M--P—0 meta l lo -oxyphosphorus compound. N--P--C a z a - o r g a n o p h o s p h o r u s compound M--P--C me ta l lo -o rganophosphorus compound. M--P--N me ta l l o - azaphosphorus compound.

1.7 61

More detailed compound classification follows established nomenclature schemes although there are frequently alternative choices which can be made.These matters are dealt with in the appropriate chapters concerned (see also Appendix II ).

Phosphorus chemistry is grouing so fast that this Table should be regarded

only as introductory and temporary. Only major groups are indicated and some

important minor groups of compounds are not included.

co CO

I

I Î

LOI o o 1—< I

•H

CO

ε O

10

co I ° a,

o oo

o M o

M

M °

il

I JaAHdsmdomis j j 3 JJ îj

| ¹ f m 0Hdo^|j j 5 j | | j

1 h h / | ε J 2 3NIHdS0Hd0™i3U J | | J | / |

L _ _ ■ ■ ■ In " I O

I L h i A 1 | 1 S3NAZWdS0Hd S | 3 | \ |

X i I S3N3Z«HdS0Hd | | .5 | \ * |

8 11 -—-Il i i i \ Γ 11 h — j ; I s I / β\

% v Ãí s I I si I 7 - o .. Ü I s , y s3N3HdsoHd\ ΛΕ i | g i / s i — Ù s i § -s / / £ i o s i S Y S3MHdS0Hd | | § | / | / 'Ί

�* χ

a .

| c o en -o CD !

i ï — J 5 i i l y "^ 1 " i º = ° s! C/5 I 10 (D CJ (0 I

g ï S3lVH-S0Hd f | | | f ~ \ | |

I I o a. I

ϊ,ΐϊίΤΓΪ! I j j j l l l J I I I |

62 1.7

REFERENCES

Section 1 .1

(1) J.R. VanWAZER, "Phosphorus Z its Compounds Vol 1, Wiley, New York 1958 (2) M. BOAS, "Robert Boyle in Eighteenth Century Chemistry", Cambridge Univ Press 1958

(3) D.R. PECK, "The History Z Occurrence of Phosphorus" in Mellor's Comprehensive

Treatise on Inorganic Z Theoretical Chemistry, Vol 8, Supp 3, Longman, London 1971.

(4) M.E. WEEKS Z H.M. LEICESTER, "Discovery of the Elements", J.Chem.Ed. Pub.Easton 1968

(5) D.E.C. CORBRIDGE, "The Structural Chemistry of Phosphorus", Elsevier,Amsterdam 1974.

Se£t ion 1^2

(1) W.F. STOWASSER, Minerals Yearbook Vol 1, US Bureau of Mines, Washington, 1988.

(2) A.J.G. NORTHOLT, R.P. SHELDON, D.F. DAVIDSON (Eds) "Phosphate Deposits of the World"

Vol 2, Cambridge Univ Press 1989.

(3) ANON "World Mineral Statistics" 1981-1985, British Geological Survey,Keyworth 1987

(4) M.C. MEW, "World Survey of Phosphate Deposits" 4 Ed, British Sulphur Corp. 1983.

(5) J.W. BRINCK, "World Resources Z Phosphorus" in Phosphorus in the Environment, Ciba

Foundation Symposium No 57, Elsevier, Amsterdam, 1978.

(6) G.D. EMIGH, "Phosphate Rock" in Industrial Minerals Z Rocks 4 Ed, Amer Inst Mining

New York, 1975.

(7) J.O. NRIAGU Z P.B. MOORE, "Phosphate Minerals", Springer-Verlag, Berlin, 1984.

(8) J.R. LEHR Z C. McCLELLAN, "Phosphate Rocks-Factors in Economic Evaluation, CENTO

Symp. Nov 1973.

(9) M. SLANSKY, "Geology of Sedimentary Phosphates", Elsevier, 1986.

(10) D.J. FISHER, "The Geochemistry of Minerals Containing Phosphorus" in The Environmen-

tal Phosphorus Handbook, Ed E.J. Griffith et al., Wiley, New York, 1973.

(11) V.E. Mc ELVEY, "Abundance Z Distribution of Phosphorus in the Lithiosphere in (10). (12) D.R. PECK, "The Utilisation of Phosphorus Minerals" in Mellor's Comprehensive

Treatise on Inorganic Z Theoretical Chemistry, Vol 8, Supp 3, Longmans, London 1971.

(13) - Proceedings of Phosphate Industrials Minerals Conference, Orlando,

Florida, Dec 1983.

(14) J.B. CATHCART "Sedimentary Phosphate Deposits of the World" in (13)

(15) A.J.G. NORTHOLT, "The Growing Contribution of Igneous Phosphate " in (13).

(16) R.P. SHELDON, "Phosphate Rock", Sei. Amer.,_246, 45 June 1982.

(17) G.H. Mc CLELLAN Z T.P. HIGNETT, "Economic Z Technical Factors in Phosphate Use" in 5

(18) J.R. LEHR, "Impact of Phosphate Rock Quality on Market Use" Ind. Minerals.,May 1984

(19) - General Bibliography - Phosphate Mining Z Production, Ind.Minerals Index 67-87

(20) M. WILLIAMS Z B. MACDONALD, "The Phosphateers" (Xmas Isle), Melbourne Univ Pressl985

(21) R.L. DAY, "Trends in the Idaho Z Western Phosphate Fields", Idaho Mines Bureau, 1973

(22) J.B. CATHCART Z D.L. SCHMIDT, "Antarctic Phosphate" US Govt.Print Office, 1977.

(23) T. MINSTER et al "Oil Shale Phosphorites" Ind. Minerals., March 1986 p 47.

(24) A.F. BLAKEY, "The Florida Phosphate Industry", Harvard Univ. Press, 1973.

(25) K. SV0B0DA "Phosphates of Tunisia" Ind.Miner. p37 Dec 1984.

(26) P. HARBEN "Phosphates of Brazil" Ind.Miner, p 35 Dec 1983.

(27) P. HENDERSON, "Inorganic Geoshemistry", Pergammon, 1982.

(28) D.S. CR0NAN, "Underwater Minerals" Chap 4, Academic Press, 1980.

(29) E. WANK, "Physical Resources of the Ocean", Sei.Amer., Sept 1969.

(30) H.D. HOLLAND, "Chemistry of the Atmosphere Z Oceans", Wiley, 1978. (31) P.R. HESSE, "Phosphorus in Lake Sediments" in (10).

(32) C.P SPENCER, "Chemical Oceanography" Ed J.P.Riley Z G.Kirrow,Acad.Press.Vol 2, 1975

(33) R.A. GULBRANDEN Z C E . ROBERS0N, "Inorganic Phosphorus in Seawater" in (10)

1.7 63

(34) E.A. THOMAS, "Phosphorus in Eutrophication" in (10)

(35) C.N. SAWYER, "Phosphorus Z Ecology" in (10)

(36) C.S. REYNOLDS, "Phosphorus ε the Eutrophication of Lakes" in (5)

(37) R.J.P. WILLIAMS, "Phosphorus in the Environment" in (5).

(38) E.J. GRIFFITHS, "Mankinds Influence on the Natural Cycle of Phosphorus" in (5).

(39) T.L. GROVE, "Phosphorus Biogeochemistry" in Encl.Earth System Sei, Vol 3, Acad P 1992

(40) J.E. RICHEY, "The P Cycle" in Biogeochemicals Z their Interactions Ed B. Bolin Z R.B. Cook, Wiley, New York, 1983.

(41) R.W. COLLINGWOOD, "The Dissipation of Phosphorus in Sewage Z Effluents" in (5). (42) R.P.G. BOWKER Z H.D. STENSEL Eds, "Phosphorus Removal from Wastewater" Noyes Datal990

(43) M.T.J. MEGANCK ε G.M. FAUP, "Enhanced Bio P Removal from Wastewaters" in Biotreatment

Systems, Vol 3 Ed D.L. Wise C.R.C. 1987.

(44) Phosphorus removal from wastewater - US 4948510 US 4956094.

Sections 1_L§_Z_I.LË_

(1) H. GOLDWHITE, "Introduction to Phosphorus Chemistry" Cambridge Univ.Press 1981

(2) J. EMSLEY Z D. HALL, "The Chemistry of Phosphorus" Harper Z Row, London, 1976.

(3) J.R. VanWazer, "Phosphorus and Its Compounds" Vol 1, Wiley, New York, 1958.

(4) D.E.C. CORBRIDGE, "The Structural Chemistry of Phosphorus", Elsevier, Amsterdam 1974.

(5) D.W.J. CRUICKSHANK, "The Role of d-Orbitals in Bonding of Phosphorus" J.C.S 5486 1961

(6) J.E. BISSEY, "Some Aspects of d-orbital Participation in Phosphorus Z Silicon Chemistry", J.Chem.Ed., 44, 95 (1967).

(7) K.A.R. MITCHELL, "The Use of Outer d-Orbitals in Bonding", Chem.Revs., 69, 157 (1969)

(8) H. KWART ε Κ. KING, "The Role of d-Orbitals in the Chemistry of Si fP Z S", Springer-Verlag, Berlin, 1977.

(9) D.A. BOCHVAR, N.D. GAMBARYN Z L.M. EPSHTEIN, "Concepts of Vacant d-Orbitals and

Differences between N and P Compounds", Russ.Chem.Revs., 660 (1976)

(10) C A . C0ULS0N, "Theoretical Studies of d-Orbital Involvement", Nature, 221, 1106 1969

(11) R. APPEL, F. KNOLL Z I. RUPPERT, "Multiple pf> -pf> Bonds " AWC 20 731 1981.

(12) A.H. COWLEY et al., "Double Bonds between Heavier Group Va Elements" IC 23 2582 1984

(13) E. FLUCK, Topics Phos.Chem., Π) 193 1980

(14) A.H. COWLEY, Polyhedron, 3, 389 1984

(15) A.H. COWLEY, "Stable Compounds with Double Bonding " Accounts Chem. Res j_7 386

1984

(16) O.J. SCHERER, "Low Cbord P - Multiply Bonded etc — " AWC 24 924 1985

(17) A.H. COWLEY, "Multiple Bonds between Main Group Elements" PS 26 327 1986

(18) D.G. GILHEANY, "Structure Z Bonding in Organophosphorus Compounds" Chapter 2 in

The Chemistry of Organophosphorus Compounds Ed F.R. HARTLEY, Wiley, Chichester, 1990

(19) W.W. SCHOELLER "Bonding Properties of Low-Coordinated P Compounds" p5 in Multiple

Bonds Z Low Coordination in P Chemistry, Ed M. REGITZ Z 0.J.SCHERRER, G.Thieme,1990.

(20) F. MATHEY, "Expanding the Analogy between P=C ε P=P — " Ace.Chem.Res.25 90 1992.

(21) K.S. PITZER, "Double Bond Rule " JACS 70 2140 1948

(22) R. MULLIKEN " " JACS 72_ 4493 1950. (23) M.W. SCHMIDT, P.N TRUONG, M.S. GORDON. JACS ]09 5217 1987

(24) W. KUTZELNIGG "Bonding ε Polarisability " AWC £3 272 1984.

(25) W.W. SCHOELLER, T. DABISCH, T. BUSCH IC 26 4383 1987

(26) L. PAULING "The Nature of the Chemical Bond" 3r d Ed Cornell Univ Press 1960

(27) E. CARTMEL ε G.W.A. FOWLES, "Valency ε Molecular Structure" 3r d Ed, Butterworths 1976

(28) R.F. HUDSON, "Structure ε Mechanism in Organophosphorus Chemistry" Acad.Press,NY 1965

(29) T.H. LOWRY ε K.S. RICHARDSON, "Mechanism ε Theory in Inorganic Chemistry" Harper ε

Row, New York, 1976.

(30) C.K. INGOLD, "Structure ε Mechanism in Organic Chemistry", Bell, London, 1967

M 1.7

(31) S.B. HARTLEY et al., "Thermochemical Properties of Phosphorus Compounds"

Quart.Revs., J_7 204 1963.

(32) G. PILCHER, "Thermochemistry of Phosphorus Compounds" pl27 Chapter 5 in (18).

Addendum An exact description or representation of the phosphoryl bond

has always been difficult, but P = 0 is stil l preferable to P—0 or P = 0 . The traditional participation of d orbitals has very recently been put in doubt and the subject is controversial. An extended discussion is outside the scope of this book but the topic has been t h o r o u g h l y r e v i e w e d (D.G. GILHEANY in The Chemistry of Organophosphorus Compounds

Vol 2 Chapter 1 , Vol 3 Chapter 1 1992, 1994 Wi ley , New York ) .