Question

How many geometric isomers can planar complex of type M(ABCD) show?

Answer 1

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-.

Geometric Isomers

The number of geometric isomers expected for common stereochemistries are as follows:

Square Planar:

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. Thus for example, cis-PtCl2(NH3)2is an anti-cancer agent (cisplatin) whereas the trans- isomer is inactive against cancer (it is toxic), and so not useful in Chemotherapy.

Answer 2

Answer : The total number of geometric isomers can be, 3

Explanation :

Geometrical isomerism or cis-trans isomerism :

The complex compound which have the same ligands in the coordination sphere but the relative position of the ligands round the central metal atom is different are called geometrical isomers and the phenomenon is called geometrical isomerism.

The complex compound having two ligands occupying the adjacent position to each other is called cis-isomer while that in which the two ligands occupying the opposite position to each other is called trans-isomer.

$[M(ABCD)]$ in this complex, A, B, C and D are different ligand and this is the example of square planer complex type. This complex ion exists in three isomeric forms.

The geometrical isomer of $[M(ABCD)]$ are shown below.