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By:-
DR. VIKRAM SINGHTANUSHREE SINGH
YEAR OF PUBLICATION-2010All rights reserved. No part of this publication may be reproduced,
stored in a retrieval system, transmitted in any form or by any means-
Electronic, Mechanical, Photocopying, Recording or otherwise, without
prior permission of the Authors and Publisher
SAVANT INSTITUTE
TM
CLASS XIICHEMISTRY
Chemistry Coordination Compounds 95
SAVANT EDUCATION GROUP E-17, East of Kailash, New Delhi – 110065. Ph.: +91-11-26224417 www.savantgroup.org
7
COORDINATION COMPOUNDS
Slide 1
§ Coordination chemistry is an important and challenging area of modern inorganic chemistry.
§ Coordination compounds find use in analytical chemistry, medicial chemistry, metallurgical processes and industry.
§ Modern concepts of chemical bonding and structure make it possible to understand the formation and functioning of such compounds.
§ Their nomenclature, properties and other structural aspects would be taken up in this chapter.
(a) Those which lose their identity in solution (double salt) (b) Those which retain their identity in solution (complexes) When crystals of carnallite are dissolved in water, the solution shows the properties of K+, Mg2+ and Cl– ions.
KCl. MgCl2. 6H2O à K+ + Mg2+ + 3Cl– + 6H2O Carnallite is a double salt.
§ A complex in which the ion carries a net positive charge is called cationic complex e.g. [Co (NH3)6]3+, [Ni(NH3)6]2+ etc.
§ A complex in which the ion carries a net negative charge is called an anionic complex, e.g. [Ag (CN)2]–, [Fe (CN)6]4–
§ A complex carrying no net charge is called a neutral complex or simply a complex, e.g.
[Ni (CO)4], [CoCl3 (NH3)3] etc.
Slide 5
Some important points
(i) Central ion or Centre of co-ordination– The cation to which one or more neutral molecules or ions are attached is called the centre of co-ordination.
(ii) Ligands– The neutral molecules or ions which are attached with the central metal ion in coordination entity are called ligands. In most of complexes a ligand acts as a donor partner i.e. it donates one (or more) electron pair to the central metal ion. The common donor atoms in ligands are nitrogen, oxygen and less common are arsenic and phosphorous etc.
(iii) Co-ordination number– It is the total number of the atoms of the ligands that can co-ordinate to the central metal ion. For example, in the complex ions [Ag(CN)2]–, [Cu(NH3)4]2+ and [Cr(H2O)6]3+ the co-ordination number of Ag, Cu and Cr are 2, 4 and 6 respectively. Similarly in the complex ion, [Fe(C2O4)3]3–, the co-ordination number of Fe is 6 because C2O4
(iv) Co-ordination sphere – The central metal ion and the ligands that are directly attached to it are enclosed in a first sphere of attraction and written in a square bracket. It is called as coordination sphere. The ionizable groups are written outside the brackets. The ions being outside the square bracket form the second sphere of attraction.
(v) Spatial arrangement of ligand atoms attached to central atom form a coordination polyhedra which can be octahedral
[Co(NH3)6]3+ i.e., or tetrahedral [Ni(CO)4]
i.e., or square planar [Pt Cl4]2– i.e., etc.
100 Coordination Compounds Chemistry
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Slide 8
(vi) Oxidation number or oxidation state – It is a number that represents the charge which the central atom or ion actually has or appears to have when combined with other atom, e.g., oxidation number of copper in [Cu(NH3)4]2+ is +2 but co-ordination number is 4.
(vii) Charge on the complex ion – The charge carried by a complex ion is the algebric sum of the charges carried by central metal ion and the ligands co-ordinated to the central metal ion. For example, the net charge on the complex ion [Ag(CN)2]- is +1 – 2 = –1.
(ii) Bidentate , tridentate , polydentate ligands: Ligands having two, three, four, five or six donor atoms are called bi, tri (or ter-) tetra (or quadri) Penta and Hexa (or sex–) dentate ligands respectively. When a ligand can bind through two donor atoms as in C2O4
2– (oxalate) it is said to be didentate. When several donor atoms are present in a single ligand the ligand is said to be polydentate.
(i) Chelating ligands form more stable complex than the monodentate analogs. This is called chelating effect.
(ii) Chelating ligands which do not contain double bonds e.g. ethylenediamine form five membered stable rings. The chelating ligands such as acetylacetonate form six membered stable ring complexes.
(iii) Ligands with large group, form unstable ring than the ligands with smaller groups due to steric hinderance.
(i) In analytical chemistry (ii) In water softening (iii) In the elimination of harmful radioactive metal, from the body (iv) In solvent extraction (v) In food preservation
102 Coordination Compounds Chemistry
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Slide 23
Nomenclature of coordination compounds
The basic rules for systematic naming the complex compounds are summarized here. (i) The positive ion is named first followed by the negative ion. (ii) When writing the name of a complex, the ligands are quoted
in alphabetical order, regardless of their charge (followed by the metal).
(iii) When writing the formula of complexes, the complex ion should be enclosed by square brackets. The metal is named first, then the coordinated groups are listed in the order: negative ligands, neutral ligands, positive ligands (and alphabetically according to the first symbol within each group).
(iv) When there are several ligands of the same kind, we normally use the prefixes di, tri, tetra, penta and hexa to show the number of ligands of that type.
(v) An exception occurs when the name of the ligand includes a number, e.g. dipyridyl or ethylenediamine. To avoid confusion in such cases, bis, tris and tetrakis are used instead of di, tri and tetra and the name of the ligand is placed in brackets.
(vi) The oxidation state of the central metal is shown by a Roman numeral in brackets immediately following its name (e.g. titanium (III).
Compounds which have the same chemical formula but different structural arrangements are called isomers, and the phenomenon is known as isomerism. Werner classified all the possible type of isomers of co-ordination compounds in the following classes.
§ Certain ligands contain more than one atom which could donate an electron pair.
§ In the NO2– ions, either N or O atoms could act as the
electron pair donor. Thus there is the possibility of isomerism. Two different complexes [Co (NH3)5 NO2] Cl2 have been prepared, each containing the NO2
This type of isomerism occurs when both cation and anion are complex. The isomerism is caused by the interchange of ligands between the two complex ions of the same complex. Examples are, (i) [Co (NH3)6] [Cr (CN)6] and [Cr (NH3)6] [Co (CN)6] (ii) [Co (NH3)6] [Cr (C2O4)3] and [Cr (NH3)6] [Co (C2O4)3]
§ When two identical groups (ligands) occupy adjacent positions, the isomer is called cis and when these are opposite to one another the isomer is called trans.
§ This isomerism is not possible for complexes with coordination number 2, 3 and tetrahedral complexes with coordination number 4.
§ However, cis-trans isomerism is quite common in square planar and octahedral complexes.
(iii) [M (AA) B2C2] type i.e. containing one symmetrical bidentate ligand e.g. [Co (en) (NH3)2Cl2]+. Its two optical isomers may be represented as follows:
Square planar complexes do not show optical isomerism because they contain a plane of symmetry but tetrahedral complexes containing unsymmetrical bidentate ligands e.g. [Ni(NH3CH2COO)2] i.e., bis(glycinato) nickel (II) shows optical isomerism.
This theory is mainly due to Pauling. It deals with the electronic structure of central metal ion in its ground state, kind of bonding, geometry (i.e., shape) and magnetic properties of the complexes. this is based on the following assumptions: (i) The central metal atom or ion (as the case may be) makes
available a number of empty s, p and d atomic orbitals equal to its coordination number. These vacant orbitals hybridise together to form hybrid orbitals which are the same in number as the atomic orbitals hybridizing together. These are vacant, equivalent in energy and have definite geometry.
Slide 56
(ii) The ligands have at least one σ-orbital containing a lone pair
of electrons. Vacant hybrid orbitals of the metal atom or ion overlap with
the filled (containing lone pair of electrons) σ-orbitals of the
ligands to form ligand à metal σ - bond (represented as Là M). This bond, which is generally known as coordinate bond
is special type of covalent bond and shows the
characteristics of both the overlapping orbitals. However, it also possesses a considerable amount of polarity because
It may be pointed out that F- ion provides a weak ligand field and is unable to pair up the unpaired electrons of the 3d orbitals. Hence six equivalent hybrid orbitals are obtained by mixing up of one 4s, three 4p and two 4d orbitals. The highly paramagnetic nature of the complex further confirms the presence of four unpaired electrons.
108 Coordination Compounds Chemistry
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Slide 67
Inner and Outer Orbital Complexes:
We have seen that in octahedral structures, the central metal atom either uses inner (n -1) d-orbitals or outer n d-orbitals for hybridization. Accordingly, the complexes are classified as follows:
When the complex formed involves the inner (n -1) d-orbitals for hybridization (d2sp3), the complex is called inner orbital complex. In this case, the electrons of the metal are made to pair up, so the complex will be either diamagnetic or will have lesser number of unpaired electrons. This type of complex is also known as low spin complex. The first three examples discussed above are inner orbital complexes. Some other examples of inner orbital complexes are [V(H2O)6]3+, [Co(NH3)6]3+, [Mn(CN)6]3-, [Co(CN)6]4-, [Co(CN)6]3-, [Fe(H2O)6]2+.
When the complex formed involves the use of outer nd-orbitals for hybridization, (sp3 d2), the complex is called outer orbital complex. The complex will have large number of unpaired electrons as the configuration of the metal remains unchanged. This type of complex is also called high spin complex. Examples:- [CoF6]3- [MnF6]3-, [FeF6]3-, [Ni (NH3)6]2+, [Fe (H2O)6]3+.
(i) Formation of [Ni (CO)4]0. Oxidation state of nickel in this complex is '0'. Its electronic configuration is [Ar] 3d84s2. Hence we have sp3 hybrid orbitals accommodate four pair of electrons from four CO molecules and the resulting tetrahedral complex is diamagnetic due to absence of unpaired electrons.
Slide 71
(ii) Formation of [Zn (NH3)4]2+. Complexes of Zn2+ are
Complexes of Ni (II), Co (II), Ag(I) etc. which have more than one co-ordination number depending on the nature of the ligand, generally do not follow the EAN rule. Some metal atoms such as Fe (III) which has its co-ordination number equal to 4 in [FeCl4]– and equal to 6 in [Fe(CN)6]3+ never follow this rule.
§ The V.B. theory does not explain the colour and spectra of complex.
§ The theory shows the number of unpaired electrons. § From this the magnetic moment can be calculated. § However., it does not explain why the magnetic moment
varies with temperature . § It does not give a quantitative interpretation of the
thermodynamic stability of coordination compounds. § It involves a number of assumptions.
§ If the ligand is a neutral molecule such as NH3, the negative end of the dipole in the molecule is directed towards the metal ion.
§ The electrons on the central metal are under repulsive forces from those on the ligands.
§ Thus the electrons occupy the d orbitals farthest away from the direction of approach of ligands.
§ In the crystal field theory the following assumptions are made: (i) Ligands are treated as point charges. (ii) There is no interaction between metal orbitals and
§ It follows that the approach of six ligands along the x, y, z, –x, –y and –z directions will increase the energy of the orbitals which point along the axes much more than it increases the energy of the dxy, dxz and dyz orbitals (which point between the axes).
§ Thus under the influence of an octahedral ligand field the d orbitals split into two groups of different energies as shown below (figure).
§ Rather than referring to the energy level of an isolated metal atom, the weighted mean of these two sets of perturbed orbitals is taken as the zero: this is sometimes called the Bari centre.
§ The difference in energy between the two d levels is given either of the symbols ∆o or 10 Dq.
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§ It follows that the eg orbitals are +0.6∆o above the average level, and the t2g orbitals are –0.4∆o below the average value as shown in figure.
§ The magnitude of ∆o depends on three factors: (i) The nature of the ligands. (ii) The charge on the metal ion. (iii) Whether the metal is in the first, second or third row of
transition elements. § Ligands which cause only a small degree of crystal field
splitting are termed weak field ligands. § Ligands which cause a large splitting are called strong field
ligands. § Most ∆ values are in the range 7000 cm–1 to 30000 cm –1.
§ In tetrahedral coordination entity formation, the d orbital splitting (see figure) is inverted and is smaller as compared to the octahedral field splitting.
§ For the same metal, the same ligands and metal-ligand distances, it can be shown that ∆t = (4 / 9) ∆0.
§ Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and, therefore, low spin configuration are rarely observed.
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CURRICULUM BASED WORKSHEET
Topics for Worksheet – I
§ IMPORTANT TERMS
§ NOMENCLATURE
§ ISOMERISM
Worksheet – I
1. Explain the following terms (a) Coordination number (b) Counter ions
2. What is a complex ion? 3. What do you understand by the term stability constant K
for a complex? 4. There are two compounds [Cr(NH3)4]2+ and [Cr(CN)4]2–
having 5.2 × 1011 and 3.2 × 1022 respectively as their K values, which complex is more stable and which ligand NH3 or CN– is a stronger ligand?
5. Write the chemical formula for the following coordinate compounds — (a) Potassium trioxalato ferrate (III). (b) Dichloro bis (ethylenediamine) copper (II). (c) Penta aquanitrosonium ion (I) sulphate. (d) Potassium hexacyanoferrate (III). (e) Tetracyanonickelate (II) ion.
6. Write IUPAC name of the following complexes: (i) [Fe (H2O)6 SO4] (ii) [Cu (en)2] (NO3)2
7. Explain the facial and meridional isomerism. 8. Explain the optical isomerism in coordination
compounds. 9. What is macrocyclic effect? 10. Calculate the oxidation state of the central metal atom
in the following coordinate compounds? (a) Na[AlH4] (b) [Ni(CO)4] (c) [PtCl2(NH3)2]2+ (d) [Co(en)2
(ONO)Cl]Cl (e) K3 [Co(ox) 3]
11. Give IUPAC name of the following complexes: (i) Na2[Fe(CN)5(NO) (ii) K2[Zn(CN)4]
12. Provide systematic names to the following complexes: (i) [Cr(NH3)6]3+ (ii) [Pt(NH3)2Cl2] (iii) [NiCl4]2–
13. Write the chemical formula of (i) Sodium tetrahydridoborate (III) (ii) Tetrahydroxozincate (II) ion
14. Account for the fact that both [Ni(CO)4] and [Ni(CN)4]2–
are diamagnetic. 15. Briefly explain the shortcomings of valence bond theory. 16. What are ambident ligands? What are their importance?
Topics for Worksheet – II
§ VALENCE BOND CONCEPT
§ CRYSTAL FIELD THEORY
Worksheet – II
1. Explain the hybridization and magnetic behavior of [Ni(CN)4]2–.
2. [NiCl4]2– is paramagnetic in nature, explain. 3. Draw figure to show splitting of degenerate d-orbitals in
an octanhedral crystal field? 4. How does the metal carbonyls gain stability although
CO is a weak donor? 5. Discuss the concept of backbonding in metal carbonyls. 1. What are the main features of the valence bond theory?
Explain briefly. 2. Briefly explain the concept of crystal field theory. 3. What are inner orbital and outer orbital coordination
entities? Explain them with examples. 4. Explain the crystal field effects and splitting up of d-
orbitals in octahedral complezes.
CURRICULUM BASED CHAPTER ASSIGNMENT
2 Mark Questions
1. What do you mean by ambidentate ligands? 2. Give two examples of coordination isomerism. 3. Giving a suitable example describe the importance of
the formation of complex compounds in: (i) The estimation of hardness of water. (ii) The extraction of a particular metal from its natural
source.
3 Marks Questions
4. Why geometrical isomerism is not found in tetrahedral complexes?
5. Write the main assumption of valence bond theory. 6. What do you mean by coordination number? Explain
with example. 7. What is an ionization isomerism? Explain it with an
example. 8. What are hydrate isomers? 9. What is the significance of ∆0?
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5 Marks Questions
10. How would you purify the impure sample of Ni? 11. Explain with examples Cis -trans geometrical isomerism. 12. Define cis and trans isomerism in complexes giving
suitable examples. 13. How would you account for the following:
(a) [Ti(H2O)6]3+ is coloured while [Sc(H2O)6]3+ is colourless,
(b) [Fe(CN)6]3– is weakly paramagnetic while [Fe(CN)6]4– is diamagnetic,
(c) [Ni(CO)4] possesses tetrahedral geometry while [Pt(NH3)Cl2] is square planner.
OR Using the valence bond approach predict the shape and magnetic character of [FeCN6)]3–. At no. of Fe = 26.
14. Briefly describe the nature of bonding in metal carbonyls.
QUESTION BANK FOR COMPETITIONS
1. Which one of the following has largest number of isomers? (a) [Ir(PR3)2H(CO)]2+ (b) [Co(NH3)5CI]2+ (c) [Ru(NH3)4Cl2]+ (d) [Co(en)2Cl2]+
2. In the compound lithium tetrahydrido-aluminate, the l igand is (a) AI+ (b) H (c) H– (d) None of these
3. The EAN of iron in [Fe(CN)6]3– is (a) 34 (b) 36 (c) 37 (d) 35
4. Coordination number of Ni in [Ni(C2O4)3]4– is (a) 3 (b) 6 (c) 4 (d) 5
5. The IUPAC name of K3[Ir(C2O4)3] is (a) Potassium trioxalatoiridium (III) (b) Potassium trioxalatoiridate (III) (c) Potassium tris (oxalato) iridium (III) (d) Potassium tris (oxalato) iridate (III)
6. The chemical formula for iron (III) hexacyanoferrate (II) is (a) Fe[Fe(CN)6] (b) Fe3[Fe(CN)6] (c) Fe3[Fe(CN)6]4 (d) Fe4[Fe(CN)6]3
7. Which one is the most likely structure of CrCl3.6H2O if 1/3 of total chlorine of the compound is precipitated by adding AgNO3? (a) CrCl3.6H2O (b) [Cr(H2O)3Cl3]. (H2O)3 (c) [CrCl2(H2O)4]Cl.2H2O (d) [CrCl(H2O)5]Cl2.H2O
8. Which one of the following will not show geometrical isomerism? (a) [Cr(NH3)4Cl2]Cl (b) [Co(en)2Cl2]Cl (c) [Co(NH3)5NO2]Cl2 (d) [Pt(NH3)2Cl2]
9. Which of the following species represent the example dsp2 – hybridization? (a) [Fe(CN)6]3– (b) [Ni(CN)4]2– (c) [Zn(NH3)4]2+
(d) [FeF6]3– 10. Which one of the following is an example of octahedral
11. Consider the following complex [Co(NH3)5CO]ClO4. The coordination number, oxidation number, number of d-electrons and number of unpaired d-electrons on the metal are respectively (a) 6,3,6,0 (b) 7,2,7,1 (c) 7,1,6,4 (d) 6,2,7,3
12. Atomic numbers of Cr and Fe are respectively 24 and 26. Which of the following is paramagnetic with the spin of the electron? (a) [Cr(CO)6] (b) [Fe(CO)5] (c) [Fe(CN)6]4– (d) [Cr(NH3)6]3+
13. Which of the following is organo-metallic compound? (a) Ti(C2H4)4 (d) Ti(OC2H5)4 (c) Ti(OCOCH3)4 (d) Ti(OC6H5)4
14. Which of the following organometallic compound is σ and π-bonded? (a) [Fe(η5 – C 5H5)2] (b) [PtCl3(η2 – C2H4)] (c) [Co(CO)5NH3]2+ (d) Al(CH3)3
15. Which of the following will exhibit maximum ionic conductivity? (a) K4[Fe(CN)6] (b) [Co(NH3)6]Cl3 (c) [Cu(NH3)4Cl2] (d) [Ni(CO)4]
17. Hybridization of Ag in the linear complex [Ag(NH3)2]+ is : (a) dsp2 (b) sp (c) sp2 (d) sp3
18. Which of the following have square planar structure? (a) [Ni(CN)4]2+ (b) [Ni(CO)4]2+ (c) [Ni(Cl)4]2+ (d) All of above
19. Which of the following is wrong statement? (a) Ni(CO)4 has oxidation number +4 for Ni (b) Ni(CO)4 has zero oxidation number for Ni (c) Ni is metal (d) CO is gas
20. IUPAC name of Na3[Co(ONO2)6] is (a) Sodium cobaltinitrite (b) Sodium hexanitrito cobaltate (III) (c) Sodium hexanitrocobalt (III) (d) Sodium hexanitritocobaltate (II)
21. Which has highest paramagnetism? (a) [Cr(H2O)6]3+ (b) [Fe(H2O)6]2+ (c) [Cu(H2O)6]2+ (d) [Zn(H2O)6]2+
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22. The number of geometrical isomers of [CO(NH3)3(NO2)3] (a) Zero (b) 2 (c) 3 (d) 4
23. In solid CuSO4.5H2O copper is coordinated to (a) 4 water molecules (b) 5 water molecules (c) One sulphate molecule (d) One water molecule
24. One mole of the complex compound Co(NH3)5Cl3, gives 3 moles of ions on dissolution in water. One mole of the same complex reacts with two moles of AgNO3 solution to yield two moles of AgCl (s). The structure of the complex is (a) [Co(NH3)3Cl3].2NH3 (b) [Co(NH3)4Cl2]Cl. NH3 (c) [Co(NH3)4Cl]Cl2. NH3 (d) [Co(NH3)5Cl]Cl2
25. Which one of the following complexes is an outer orbital complex? (Atomic nos : Mn = 25; Fe = 26; Co = 27; Ni = 28) (a) [Co(NH3)6]3+ (b) [Mn(CN)6]4– (c) [Fe(CN)6]4– (d) [Ni(NH3)6]2+