Question

Which of the following coordination complex ions exhibits structural asymmetry leading to optical isomerism?

• A. \(\text{trans}-[\text{Co}(\text{en})_2\text{Cl}_2]^+ \)
• B. \(\text{cis}-[\text{Co}(\text{en})_2\text{Cl}_2]^+ \)
• C. \([\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+ \)
• D. \([\text{Cr}(\text{NH}_3)_5\text{Cl}]^{2+} \)

Hint:

Octahedral metal configurations of the general type \([\text{M}(\text{AA})_2\text{X}_2]\) with bidentate chelating rings display optical activity exclusively in their cis arrangements.

Answer 1

The Correct Option is B

Concept: A coordination complex displays optical activity if it forms non-superimposable mirror images. This structural condition requires the compound to lack an internal plane of symmetry (\(\sigma\)) and a center of inversion (\(i\)).

Step 1: Evaluate internal geometric symmetry elements.
The \(\text{trans}-[\text{Co}(\text{en})_2\text{Cl}_2]^+\) configuration distributes its ligands directly opposite each other across a central inversion plane, making it symmetrical and optically inactive. In the \(\text{cis}-[\text{Co}(\text{en})_2\text{Cl}_2]^+\) isomer, the bidentate ethylenediamine ligands are placed adjacent to each other. This geometry removes all internal symmetry planes, resulting in a chiral core that forms stable \(d\)- and \(l\)- mirror image pairs.