What is isomerism in coordination compound according to bsc 2nd year?
Answers
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.
1. Stereoisomerism.
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 isomers are possible for both square planar and octahedral complexes, but not tetrahedral.
Optical isomers are 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 the cis- and trans- geometric isomers. In 1911, the first resolution of optical isomers was reported by Werner and King for the complexes cis-[CoX(NH3)(en)2]2+, where X=Cl- or Br-.
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
[Cr(H2O)6]Cl3 violet
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.