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List various types of isomerism possible for coordination compounds, giving an example of each.

List various types of isomerism possible for coordination compounds, giving an example of each.


1 Answers

Sumit Majumdar IIT Delhi
askIITians Faculty 137 Points
6 years ago
Dear student,
Two or more different compounds having the same formula are called isomers. Two principal types of isomerism are known among coordination compounds. Each of which can be further subdivided.
a) Geometrical isomerism
b) Optical isomerism
2.Structural Isomerism.
a) Coordination isomerism
b) Ionisation isomerism
c) Hydrate isomerism
d) Linkage isomerism
1. Stereoisomers
Stereoisomers have the same atoms, same sets of bonds, but differ in the relative orientation of these bonds.
Ignoring special cases involving esoteric ligands, then:
Geometric isomersare possible for both square planar and octahedral complexes, but not tetrahedral.
Optical isomersare possible for both tetrahedral and octahedral complexes, but not square planar.
The earliest examples of stereoisomerism involve complexes of Co(III). In 1889, Jorgensen observed purple and green salts of [CoCl2(en)2]+, which Werner later correctly identified as thecis-andtrans-geometric isomers. In 1911, the first resolution of optical isomers was reported byWernerand King for the complexes cis-[CoX(NH3)(en)2]2+, where X=Cl- or Br-.
Geometric Isomers
The number of geometric isomers expected for common stereochemistries are as follows:
Square Planar:
Compound type No. of isomers
Ma2b22 (cis- and trans-)
Mabcd3 (use cis- and trans- relations)
here a, b, c, and d refer to monodentate ligands.
A number of examples of these types have been isolated and characterised and they show very different chemical and biological properties.
Compound type No. of isomers
Ma4b2 2 (cis- and trans-)
Ma3b32 (fac- and mer-)
MAA2b23 (2*cis- and 1 trans-)
here a, and b, represent monodentate ligands and AA is a bidentate ligand.
In the second example, new labels are introduced to reflect the relative positions of the ligands around the octahedral structure. Thus; placing the 3 groups on one face of the octahedral gives rise to thefacial isomer and placing the 3 groups around the centre gives rise to themeridional isomer.

Optical Isomers

Optical isomers are related as non-superimposable mirror images and differ in the direction with which they rotate plane-polarised light. These isomers are referred to as enantiomers or enantiomorphs of each other and their non-superimposable structures are described as being asymmetric.
Various methods have been used to denote the absolute configuration of optical isomers such as R or S, Λ or Δ or C and A. The IUPAC rules suggest that for general octahedral complexes C/A scheme is convenient to use and that for bis and tris bidentate complexes the absolute configuration be designated Lambda Λ (left-handed) and Delta Δ (right-handed).
Priorities are assigned for mononuclear coordination systems based on the standard sequence rules developed for enantiomeric carbon compounds by Cahn, Ingold and Prelog (CIP rules). These rules use the coordinating atom to arrange the ligands into a priority order such that the highest atomic number gives the highest priority number (smallest CIP number).
The reference axis for an octahedral centre is that axis containing the ligating atom of CIP priority 1 and the trans ligating atom of lowest possible priority (highest numerical value). The atoms in the coordination plane perpendicular to the reference axis are viewed from the ligand having that highest priority (CIP priority 1) and the clockwise and anticlockwise sequences of priority numbers are compared. The structure is assigned the symbol C or A, according to whether the clockwise (C) or anticlockwise (A) sequence is lower at the first point of difference. In the example shown above this would be C.
octahedral chiral complexes
2. Structural Isomers
There are several types of this isomerism frequently encountered in coordination chemistry and the following represents some of them.
  • a) Coordination isomerism: where compounds containing complex anionic and cationic parts can be thought of as occurring by interchange of some ligands from the cationic part to the anionic part.
one isomer [Co(NH3)6] [Cr(C2O4)3]
another isomer [Co(C2O4)3] [Cr(NH3)6]
  • b) Ionisation isomers: where the isomers can be thought of as occurring because of the formation of different ions in solution.
one isomer [PtBr(NH3)3]NO2-> NO2- anions in solution
another isomer [Pt(NO2)(NH3)3]Br -> Br- anions in solution
Notice that both anions are necessary to balance the charge of the complex, and that they differ in that one ion is directly attached to the central metal but the other is not. A very similar type of isomerism results from replacement of a coordinated group by a solvent molecule (Solvate Isomerism). In the case of water, this is called Hydrate isomerism.
  • c) Hydrate isomerism: the best known example of this occurs for chromium chloride "CrCl3.6H2O" which may contain 4, 5, or 6 coordinated water molecules.
[CrCl2(H2O)4]Cl.2H2O bright-green
[CrCl(H2O)5]Cl2.H2O grey-green
These isomers have very different chemical properties and on reaction with AgNO3to test for Cl- ions, would find 1, 2, and 3 Cl- ions in solution respectively.
  • d)Linkage isomerismoccurs with ambidentate ligands. These ligands are capable of coordinating in more than one way. The best known cases involve the monodentate ligands SCN- / NCS- and NO2-/ ONO-.
For example:
[Co(ONO)(NH3)5]Cl the nitrito isomer -O attached
[Co(NO2)(NH3)5]Cl the nitro isomer - N attached.
Thanks & Regards
Sumit Majumdar,
askIITians Faculty
Ph.D,IIT Delhi

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