Question:

Which one of the following octahedral complexes will not show geometric isomerism? (A and B are monodentate ligands)

Updated On: Oct 17, 2023

[MA₂B₄]
[MA₃B₃]
[MA₄B₂]
[MA₅B]

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The Correct Option is D

Solution and Explanation

Geometric isomerism occurs in octahedral complexes when there are at least two different types of ligands or when there is a bidentate ligand present. Geometric isomerism is not possible when all ligands are the same type and are monodentate. In such cases, the ligands can only occupy adjacent positions in the octahedral coordination sphere.
Among the options given:
(1) [MA₂B₄] has two types of ligands (A and B), so it can exhibit geometric isomerism.
(2) [MA₃B₃] has two types of ligands (A and B), so it can exhibit geometric isomerism.
(3) [MA₄B₂] has two types of ligands (A and B), so it can exhibit geometric isomerism.
(4) [MA₅B] has only one type of ligand (A), and they are all monodentate. Therefore, it will not show geometric isomerism.

So, the correct answer is (D): [MA₅B].

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Concepts Used:

Isomerism in Coordination Compounds

Isomers in Coordination Compounds

Stereoisomers occur when the ligands have the same bonds, but the bonds are in different orientations relative to one another. Structural isomerism occurs when the bonds are different. Isomers are distinct compounds that can have different physical properties such as color, crystal structure, and melting point.

Cis and Trans Stereoisomers

In octahedral complexes—with four of one ligand and two of another—and square planar complexes—with two of one ligand and two of another—there are two different arrangements of the same atoms with the same bonds. These different arrangements are called cis and trans. In cis molecules, the two ligands are on the same side of the complex. In trans molecules, the similar ligands are on the opposite sides of the molecules, meaning the bond angle is 180 degrees.

Optical Stereoisomers

Optical isomerism occurs when a molecule is not superimposable with its mirror image.

Fac and Mer Stereoisomers

When three identical ligands occupy one face of an octahedron, the isomer is said to be facial or fac. If these three ligands and the metal ion are in one plane, the isomer is said to be meridional or mer.