Question

[M$a_2$$b_2$$c_2$] has how many geometrical isomers?
(A) 5
(B) 4
(C) 3
(D) 2

Answer 1

Hint: To answer this question, you must recall the concept of isomerism in coordination compounds. Isomerism shown by coordinate compounds are broadly of two types, namely, structural isomerism and stereoisomerism. Geometrical isomerism is a type of stereoisomerism. The broad categories of geometrical isomers are cis and trans.

Complete step by step solution:
First we must understand the concept of geometrical isomerism. Geometrical isomerism is said to be exhibited when molecules have the same empirical formula but the way in which the atoms are arranged is different. They generally differ in the atom to atom bonds. It arises in coordination compounds when they are heteroleptic, i.e. they have more than one types of ligands.
The given compound has six ligand molecules attached to the central metal atom. Considering all these ligands to be monodentate, the given compound is octahedral in shape. The number of geometrical isomers formed would depend on the position of the ligand molecules with respect to each other. Geometrical isomers can be either cis isomers or trans isomers.
In the given compound, there are two atoms each of three different ligands. Thus, we can say that each pair of the same ligand can be arranged as either cis or trans. There will be a total of 5 geometrical isomers formed.
The correct answer is A.

Note:
Structural isomers are those isomers which have similar molecular formula but different structural formula. The arrangement of the atoms differs in structural isomers having no kind of reference with the spatial arrangement of the molecule. On the other hand, stereoisomers are those isomers which have the same molecular formula, similar structural formula but different spatial arrangement of the bonded atoms.