Question

Fac and Mer isomerism is associated with which of the following general formulas?
A. $[M{(AA)_2}]$
B. $[M{(AA)_3}]$
C. $[MABCD]$
D. $[M{A_3}{B_3}]$

Answer 1

Hint: First we need to understand the basics of geometrical isomerism. After that, we can easily differentiate between a fac-isomer and a mer-isomer. Geometrical isomerism is a type of isomerism that is generally observed in heteroleptic complexes.

Complete step by step answer:
Geometrical isomerism occurs due to different feasible geometrical arrangements of the ligands. When the two same groups engage adjacent positions, then the isomer is called cis-isomer, and when they are arranged opposite to one another, then the isomer is called trans-isomer.
As we know that the octahedral complexes also show cis and trans-isomerism due to the difference in the spatial arrangements of the ligands.
 In cis-form, when similar groups utilize adjacent positions at the corners of one of the octahedral faces then it is known as facial or fac-isomer while in trans-form, when the position of a donor atom is around the meridian of the octahedron then it is known as meridian or mer-isomer.
Now, we will refer to the formula of the compound to check which of these will exhibit both fac and mer-isomerism.
For the formula, $[M{A_3}{B_3}]$ there are three A ligands present on one side of the triangular face and three B ligands that are present on the opposite side of the triangular face. Thus, it exhibits both Fac and mer-isomers.
Now, let’s draw the fac and mer-isomerism structure of the following formula $MA_3B_3$

And hence the correct option is D.
Note:
We must know that the geometrical isomers are molecules that are packed in spatial arrangements with respect to one another due to the occurrence or rising of a ring structure or a double bond.