Chapter 6 Block

Chapter 6 : An Introduction to the Chemistry of D-Block Element

∙ D-block elements : consists of 38 elements (metals) from G3 to G12.∙ Transition elements : d-block element that can form ≥ 1 stable ion with a partially filled d-orbitals.

Element Ti V Cr Mn Fe Co Ni CuProton no. 22 23 24 25 26 27 28 29Electronicconfiguration

[Al]3d24s2

[Al]3d34s2

[Al]3d54s1

[Al]3d54s2

[Al]3d64s2

[Al]3d74s2

[Al]3d84s2

[Al]3d104s1

∙ Electronic configuration [Al]3d5s1 & 3d104s1 is more stable than [Al]3d4s2 & [Al]3d9s2 for Cr & Cu respectively. - half-filled & fully-filled d-orbitals are more stable.

Atomic radius (nm)

0.145 0.132 0.125 0.124 0.124 0.125 0.125 0.128∙ Atomic radius remains almost constant. - nuclear charge ↑ , atomic radius ↓ both factors cancel off - shielding effect ↑, atomic radius ↑ each other

Melting point ( ºC )

1680 1900 1890 1240 1540 1500 1450 1080∙ Generally, melting point ↓ - e- in the d-orbitals are being paired up & do not participate in metallic bonding

Boiling point ( ºC )

3260 3400 2645 2040 2900 2875 2800 2585∙ Generally, boiling point ↓ - e- in the d-orbitals are being paired up & do not participate in metallic bonding

∙ Transition elements have high boiling points & melting points - presence of strong metallic bonds in solid lattice. -small energy difference between 3d & 4s orbitals (enable e- to delocalise).

Density (gcm-3) 4.50 6.11 7.04 7.43 7.87 8.90 8.91 8.95∙ Density ↑ gradually - atomic mass ↑, but atomic radius remains almost unchanged.∙ All transition elements has high densities (small atomic size).

First ionization energy (kJmol -1)

611 648 653 716 762 757 736 745∙ Remains almost constant - nuclear charge ↑ cancel off - shielding effect ↑ each other

1. Chemical properties of transition elements

i. Small energy difference between 3d & 4s orbitals enable e- from these 2 orbitals to be used for bonding → all elements have ≥ 1 oxidation states.

Element Oxidation Number+1 +2 +3 +4 +5 +6 +7

Ti TiCl2 TiO3

TiCl3

TiO2

TiCl4

V ns V2O3

VCL3

ns V2O5

Cr ns Cr2O3

CrCl3

ns ns CrO3

Mn MnOMnCl2

MnCl3 MnO2 ns ns MnO7

Fe FeOFeCl2

Fe2O3

FeCl3

ns ns K2FeO4

Co CoOCoCl2

Co2O3 ns

Ni NiONiCl2

ns NiO2

Cu Cu2OCu2Cl2

CuOCuCl2

* Table shows the move stable oxidation numbers with examples of oxides & chlorides of the transition elements ◦ ns → not stable

ii. Removal of e- from 3d & 4s orbitals are equally feasible → variable oxidation states depend on no. of e- in these 2 orbitals (+2 oxidation state becomes more stable towards the end of the series.)

iii. Higher oxidation state (+6) are not formed after manganese → 3d e- are being paired up & have less tendency to be used in bonding.

iv. Transition elements in carbonyl (carbon monoxide) complexes have an oxidation state of 0.

v. Transition elements with : ∙ high oxidation states : oxidising agent → form acidic oxides ∙ low oxidation states : reducing agent → form basic oxides

vi. The higher oxidation states ( ≥ 4) do not exist in the form of free aqueous ions - examples: Ti2+, V2+, [ Fe(H2O)6]2+ …

vii. This is because their high charge density would polarise the water molecules that are coordinated to them resulting in the formation of oxo-anions.-Example: [V(H2O)6]4+ + H2O → [V(H2O)5(OH)]3+ + H3O+

Relative stability of the +2 & +3 oxidation state i. All transition metals, except vanadium & copper, dissolve in dilute

HCl to form M2+ / M3+ ions.ii. Determine the stability of M2+/M3+ ions by comparing Eө of M2+/M &

M3+/M ions with Eө = 0.00V (for acidic solution) & Eө = +1.23V (in O2).iii. Positivity of Eө value & stability of ions (there is extra stability in the

half-filled 3d orbitals).iv. Consider the standard electrode potentials ( Eө ) for the following system:

M3+ (aq) + e- ↔ M2+ (aq)

System Eө / VCr3+ + e- ↔ Cr2+ -0.41Ti3+ + e- ↔ Ti2+ -0.37V3+ + e- ↔ V2+ -0.24

Fe3+ + e- ↔ Fe2+ +0.77Mn3+ + e- ↔ Mn2+ +1.51Co3+ + e- ↔ Co2+ +1.81

* Oxygen state : O2(g) + 4H+(aq) +4e- ↔ 2H2O(l) Eө = +1.23V

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

Ti V Cr Mn Fe Co

Elements

Eq/V

→ For ions whose Eө (m3+ / m2+) values are higher than +1.23V, the +2 oxidation state will be more stable in the presence on O2.

→ For ions whole Eө (m3+ / m2+) values are lower than +1.23V, the +3 oxidation state will be more stable in the presence of O2.

Eq= +1.23V

2. Coloured Ions

Transition metals → Produce coloured ions in aqueous solution. → Exist as coloured compound.Example:

Ions ColourSc3+ *ColourlessTi3+ PurpleV3+ Green

Cr3+, CrO42-, Cr2O7

2- Green, yellow, orangeMn2+, MnO4

2- Pinkish, purpleFe2+, Fe3+ Green, brown

Co2+ PinkNi2+ GreenCu2+ BlueZn2+ * Colourless

I. Overlapping of the d-orbital in central metal with ligand causes the 3d subshell of the metal to have different energy level.

II. Example: [Cu(H2O)6]2+

4

*∆E = energy gap between two energy groups of d-orbital.*∆E corresponds to wavelength of certain region in visible light.

III. On transcending to a higher energy level (d-d transition), the electrons absorb a light photon of a particular wavelength (same energy as in between d-orbitals).

IV. The ions filter out this particular wavelength and appears coloured.V. Different ions exhibit different colours:

Different energy levels → different wavelengths absorbed → different colours exhibited Example: [Cr(H2O)6]3+ → red light absorbed → appears green

VI. Colours can change when:

(a) Oxidation state change (b) The ligands differ (c) Presence of water molecules

Fe2+ (light green)Fe3+ (brown)

MnO4- (purple)

Mn2+ (pale pink) Cr2O4

2- (orange)CrO4

2- (yellow)

[Cu(H2O)6]2+ (blue)[CuCl4]2- (yellow)[Cu(NH3)4]2+ (dark blue)

[Fe(H2O)6]2+ (green)[Fe(CN)]4- (yellow)

[Fe(H2O)6]3+ (brown)[Fe(SCN)( H2O)5]3+ (red)

[Cu(H2O)4]2+ (blue)Anhydrous CuSO4 (white)

Hydrated Co2+ (pink)Anhydrous Co2+ (blue)

∆E

VII. Ions with completely filled or completely empty subshells are colourless.Example:

Electronic configuration 3d – orbitals

Sc3+ [Ar]

Zn2+ [Ar] 3d10

*Sc3+ has no d-electrons to be promoted to higher energy level, whereas in Zn2+ there aren’t any gaps to promote an electron into.

3. Complex Ions

(a) Transition metal complexes consist of a transition metal atom or ion surrounded by ligands.

Ligands : ion or molecules with lone pair of electrons to form dative or coordinate bonds with the highly charged central metal ion.

: Example (i) water, H2O (ii) Cyanide, CN- (iii) Ammonia, NH3

(b) Transition metal are suitable central ions because

- can form ions with a small size and a high charge density

- have empty orbitals to accommodate the lone pairs of electrons from a ligand

(c) Types of ligands:

(i) monodentate ligands

- only 1 donor atom

- form only 1 dative bond with central metal ion

- Examples: H2O (aqua), NH3 (amine), CO (carbonyl), Cl- (chloro)

(ii) bidentate ligands

- 2 donor atoms

- form 2 dative bonds with central metal ion

- Example: Ethanedioate ion

NH3

NH3

H3N

H3N

(iii) hexadentate ligands

- 6 donor atoms

- form 6 dative bonds with central metal ion

- Example: EDTA

(iv) polydentate ligands

- 2 or more donor atoms

- form ring structure with central metal ion

- known as chelates or chelate compounds

- bidentate ligands & hexadentate ligands are included in poly dentate ligands

(d) Coordinate number

- the number of coordinate bonds formed between the ligands & the central metal ion

(not the number of ligands)

- determine the shape of transition metal complexes.

(i) complexes with coordination number of 2 are linear. Example:

- diamminesilver(I), [Ag(NH3)2]+

[H3N → Ag ← NH3]+

(ii) complexes with coordination number of 4 are either tetrahedral or square planar.

Examples:

- tetraamminecopper(II), [Cu(NH3)4]2+ (square planar)

- tetrachlorocobaltate(II), [CoCl4]2- (tetrahedral)

2-

2+

C

Co

Cl

Cl

Cl

Cl

NH3

NH3H3N

H3N

(iii) complexes with the coordinate number of 6 are octahedral. Example:

- tetraamminedichlorochromium(III), [Cr(NH3)4Cl2]+

(e) Almost all transition metal ions form complexes with water molecules when dissolved. Example: [Fe(H2O)6]2+ (light green)

Nomenclature of complex ions

Types of complexes:

- anionic complexes

- cationic complexes

- neutral complexes

Example: [Co(NH3)5Cl]Cl2 →[Co(NH3)5Cl]2+2Cl-

cation anion

Name: Pentaamminechlorocobalt(III) chloride

Ligands central metal

Nomenclature rules :

1. Cation is named before anion. 2. Ligands are named before central metal ion.

ligands → metal ion → anion

cation

3. Anionic ligands end in letter “o”. (Example: chloro- (Cl-), cyano- (CN-), oxo- (O-)) – For neutral ligands, the names of molecules will be used, except:

* H2O (aqua)

* CO (carbonyl)

* NH3 (ammine)

4. Number of each ligand is specified by the Greek prefix, for example: di, tri, tetra.

5. Ligands are named in alphabetical order, ignoring any numerical prefixes.

Cr

C

C

6. Bis-, tris- and tetrakis- will be used to replace di, tri, tetra if the ligand already has prefixes with the ligand name enclosed in parentheses ( ).

For example: [Fe(C2O4)3]3-, tris-ethanedioateferrate(III)

7. Oxidation state of the central metal ion is written in Roman numerical enclosed in ( ).

8. For neutral and cationic complexes, normal name of the metal is used.

9. In anionic complexes, the suffix –ate is added to the name of the metal.

Examples:Metal Name Metal Name

Aluminium Aluminate Chromium ChromateCobalt Cobaltate Vanadium Vanadate

Manganese Manganate Iron FerrateNickel Nickelate Copper CuprateZinc Zincate Silver Argentate

Examples:Types of

complexesFormula Name

Neutral K2[CoCl4] Potassium tetrachlorocobaltate(II)K3Fe(CN)6

NH4[Cr(NH3)2(SCN)4][Pt(NH3)6]Cl4

[Co(H2NCH2CH2NH2)3]2(SO4)3

(NH4)2[Ni(C2O4)2(H2O)2]Cationic [Ag(NH3)2]+ Diamminesilver(I) ion

[PtCl2(NH3)4]2+

[Ni(H2NCH2CH2NH2)3]2+

[Cr(NH3)5Cl]2+

[V(H2O)4O2]+

Anionic [CuCl4]2- Tetrachlorocuprate(II) ion[Fe(C2O4)3]3-

[Fe(CN)6]4-

[Cr(C2O4)3]3-

[Ni(EDTA)]2-

4. Catalyst

- Provides alternative route with lower EA.

- More effective collisions which increases the rate of reaction.- Transition elements and their compounds are important catalyst due to the ability to exhibit

variable oxidation states and the availability of empty orbitals in valence shell.

Industrial process CatalystHaber process from manufacturing ammonia

N2 + 3H2 → 2NH3 Fe or Fe2O3

Ostwald process for the manufacture of nitric acid from ammonia

4NH3 + 5O2 → 4NO + 6H2O2NO(g) + O2(g) → 2NO2(g)

Pt

Contact process for the manufacture of sulphuric acid2SO2 + O2 → 2NO2

V2O5

Hydrogenation of alkenes and the ‘hydrogenation’ of fats in the margarine industry

Ni or Pt

Polymerisation of ethane TiCl3 / R3Al (Ziegler-Natta)

Catalytic converters fitted in car exhaust Pt + Rh + Pd

Reaction of persulphate ions and iodide ionsS2O8

2-(aq) + 2I-(aq) → 2SO42-(aq) + I2(ap)

Fe2+ or Fe3+

Homogeneous Catalysis

- Exists in same phase (physical state) as the reactants.- Many transition metal ions are catalyst because the readily interconvert between different

oxidation state, 4s and 3d orbitals are close in terms of energy, can be easily oxidised or reduced, useful as redox catalysts.

- Forms an intermediate species with reactants, regenerated at the end of the reaction.- Example:

Reaction between peroxodisulphate ion, S2O82-, and iodide ion, I- with Fe3+ as catalyst,

Equation for reaction: S2O82- + 2I- →2SO4

2- + I2

Fe3+ oxidizes I- to I2 while is itself reduced to Fe2+.

Redox reaction: 2I- + 2Fe3+ → I2 + 2Fe2+

The intermediate species, Fe2+ ions, then reduces S2O82- ions to SO4

2- while Fe3+ is regeneratedS2O8

2- + 2Fe2+ → 2SO42- + 2Fe3+

Heterogeneous Catalysis

- Exist in different phase with reactants.- Catalyst is usually solid while reactants are usually gases or liquids.- Catalysis reaction occurs on the surface of solid catalyst.- Transition elements are effective as they have many empty or partially-filled 3d and 4s orbitals

to form temporary bonds with reactant molecules.- Mechanism for heterogeneous catalysis:

Adsorption of reactants onto the surface of the catalyst - Reactants adsorbed (physically or weakly bonded chemically) to the surface of the catalyst. Breaking and formation of bonds Desorption of products from the surface of the catalyst

- Adsorption activate the reaction, increases the concentration of the reactants at the surface, allows correct orientation for effective collisions to occur and weakens the original intramolecular bonds within the reactant molecules.

- The bond must be strong enough to weaken reactant molecule bonds but weak enough to allow new bonds to form and the products escape from the surface of catalyst.

Example: Hydrogenation of ethane catalysed by nickelNi

CH2 = CH2 + H2 → CH3 – CH3

(i) Both ethane and hydrogen molecules are adsorbed onto the Ni surface.

(ii) The H-H and C=C bonds are weakened to form bonds with the metal surface.

(iii) One H atom diffuses close to the bonded carbon, C-H bond is formed. That end of the original ethene now breaks free of the metal surface.

(iv) Another C-H bond is formed. The product ethane desorbs from the metal surface.

5. Isomerism in Complexes The existence of two or more different compounds having the same molecular formula is called

isomerism.

3 types of isomerism geometrical isomerism

optical isomerism

structural isomerism

Geometrical Isomerism

Geometrical isomerism is shown by :

I ) cis and trans isomers

(a) Square planar complexes with formula of Ma2b2 (a & b are monodentate ligands).

Example: Diamminedichloroplatinum (II), [Pt(NH3)2Cl2]

(b) Octahedral complexes with the formula of Ma4b2

Example: Tetraamminedichlorochromium (III), [Cr(NH3)4Cl2]+

(c) Octahedral complexes with the formula of M(x-x)2b2 ((x-x) are bidentate ligands).

Example : Bis-ethane-1,2diamminedichlorochromium (III) , [Cr(NH2CH2CH2NH2)2Cl2]+

o Cis-isomer : two b’s are on the same side

o Trans-isomer : two b’s are on the opposite side

II) fac and mer isomer

Octahedral complexes with the formula of Ma3b3

Example: Triamminetrichlorochromium (III), [CrCl3(NH3)3]

o Fac-isomer : the three identical groups ( a or b ) are in the same face of the octahedron.

o Mer-isomer : the three identical groups ( a or b ) are not found in the same face of the octahedron.

Optical Isomerism

i. Optical isomerism occurs in octahedral complexes which do not have a plane of symmetry.

ii. Optical isomer occurs in pairs . One is the mirror image of the other and they are not superimposable.

iii. Optical isomers are also called enantiomers . They are optically active.

iv. Enantiomers dextro(+): rotate plane polarised light in the clockwise direction.

laevo(-): rotate plane polarised light in the anti-clockwise direction.

V. Complexes with the formula of m(x-x)3

Example : Tris-ethanedioatecobaltate (III), [Co(C2O4)3]3-

vi. Complexes with the formula of M(x-x)2b2

Example : Bis-ethane-1,2-diamminedichlorochromium (III), [Cr(NH2CH2CH2NH2)2Cl2]+

vii. EDTA complexes ( all EDTA complexes are chiral )

Example : [Ni(EDTA)]2-

Structural Isomerism

Occurs in complexes having the same molecular formula but are different with respect to the types of ligands that are bonded to the central ion.

Example : CrCl3.6H20

Isomer formula Colour No. of moles of aqueous ion

I [Cr(H2O)4Cl2]+.Cl-.2H2O Dark Green 2

II [Cr(H2O)5Cl]2+.2Cl.H2O Light Green 3

III [Cr(H2O)6]3+.3Cl- Purple 4

Only the Cl- ions that are not bonded to the central metal ion can be subjected to reaction/ precipitation. [ Example : in isomer III , 3 Cl- ions can react ]

Isomer Formula Structure of complex ions

I [Cr(H2O)4Cl2]+.Cl-.2H2O

II [Cr(H2O)5Cl]2+.2Cl.H2O

III [Cr(H2O)6]3+.3Cl-

Table : Structure formulae of three isomers of CrCl3.6H20 .

6. The Uses of Chromium, Cobalt, Manganese and Titanium

Element Characteristic UsesTitanium Same mechanical strength

as steel, but is lighter Does not corrode

Used in the making of aircraft body, space capsules and nuclear reactors

Non-toxic Does not darken when

exposed to air containing hydrogen sulphide

Used as white pigments in paints

Used as ‘fillers’ for plastics and rubbersChromium Used to harden steel and to increase its

resistance to corrosion Electroplating Pigments in paint (chrome leather,

chrome yellow, chrome green) Strong oxidizing agent Stainless steel is an alloy of steel

consisting chromium, nickel, and carbon Alloy of chromium, vanadium tungsten is

used in high speed cutting toolsCobalt Used in the making of blue gemstones

Cobalt (II) oxides are additives in glass industry to give blue colour

Aluminium cobalt is a constituent of blue pigments

Vitamine B12 molecules is a big molecule containing a cobalt atom

Alloy of cobalt and samarium (Sm) is used to make permanent magnet

Manganese Decolourise glass Form brown glazes on pottery Compounds of manganese are oxisiding

agents Alloy of manganese and steel is used to

make high speed cutting tools and railway points