Isomerism in Coordination Compounds
Two or more different compounds having the same formula are called isomers.
Two types of isomerism are known among coordination compounds. These are:-
b) Optical isomerism
a) Coordination isomerism
b) Ionisation isomerism
c) Hydrate isomerism
Stereoisomers have the same atoms, same sets of bonds, but differ in the relative orientation of these bonds.
The number of geometric isomers expected for common stereochemistries are as follows:
Compound type No. of isomers
Ma2b2 2 (cis- and trans-)
Mabcd 3 (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.
for example, cis-PtCl2(NH3)2 is an anti-cancer agent (cisplatin) whereas the trans- isomer is inactive against cancer (it is toxic)
cis- and trans- refer to the position of 2 groups relative to each other. In the cis- isomer they are "next to each other" i.e. at 90 degrees in relation to the central metal ion, whereas in the trans- isomer they are "opposite each other", i.e. at 180 degrees relative to the central metal ion.
M ----b a----M----b
here a, and b, represent monodentate ligands and AA is a bidentate ligand.Compound type No. of isomers
Ma4b2 2 (cis- and trans-)
Ma3b3 2 (fac- and mer-)
MAA2b2 3 (2*cis- and 1 trans-)
Optical IsomersOptical 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.
The absolute configuration of [Co(en)3]3+
The two isomers have identical chemical properties and just denoting their absolute configuration does NOT give any information regarding the direction in which they rotate plane-polarised light. This can ONLY be determined from measurement and then the isomers are further distinguished by using the prefixes laevo ((-) or l) and dextro ((+) or d) depending on whether they rotate left or right.
2. Structural IsomersThere 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.
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.one isomer [PtBr(NH3)3]NO2 -> NO2- anions in solution
another isomer [Pt(NH3)3(NO2)]Br -> Br- anions in solution
- 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.
These isomers have very different chemical properties and on reaction with AgNO3 to test for Cl- ions, would find 1, 2, and 3 Cl- ions in solution respectively.[CrCl2(H2O)4]Cl.2H2O bright-green
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