Question

Indicate the complex/complex ion which did not show any geometrical isomerism :

• A. [CoCl\(_2\)(en)\(_2\)]
• B. [Co(CN)\(_5\)(NC)]\(^{3-}\)
• C. [Co(NH\(_3\))\(_4\)Cl\(_2\)]\(^+\)
• D. [Co(NH\(_3\))\(_3\)(NO\(_2\))\(_3\)]

Hint:

For octahedral complexes of the type [MX\(_n\)Y\(_{6-n}\)], remember these rules for geometrical isomerism:
\textbf{[MA\(_6\)] and [MA\(_5\)B]}: No geometrical isomers.
\textbf{[MA\(_4\)B\(_2\)] and [M(AA)\(_2\)B\(_2\)]}: Show cis-trans isomerism.
\textbf{[MA\(_3\)B\(_3\)]}: Shows fac-mer isomerism.
\textbf{[MA\(_2\)B\(_2\)C\(_2\)]}: Can show multiple isomers.
This quick check can save a lot of time in exams.

Answer 1

The Correct Option is B

Step 1: Understanding the Question
The question asks to identify which of the given coordination complexes does not exhibit geometrical isomerism. Geometrical isomerism arises when ligands occupy different spatial positions around the central metal ion. All given complexes are octahedral.
Step 2: Analyzing Each Option for Geometrical Isomerism
(A) [CoCl\(_2\)(en)\(_2\)]\(^+\): This complex is of the type [M(AA)\(_2\)B\(_2\)], where AA is a bidentate ligand (en) and B is a monodentate ligand (Cl). Such complexes can exist in two geometrical forms: \textit{cis} (the two Cl ligands are adjacent) and \textit{trans} (the two Cl ligands are opposite). Thus, it shows geometrical isomerism.
(B) [Co(CN)\(_5\)(NC)]\(^{3-}\): This complex is of the type [MA\(_5\)B]. In an octahedral geometry, there is only one possible arrangement for this type of complex. Replacing any of the five 'A' ligands (CN) with the single 'B' ligand (NC) results in the same structure, as all positions are equivalent relative to the other five identical ligands. Therefore, it does not show geometrical isomerism. Note that CN\(^-\) and NC\(^-\) represent linkage isomerism, not geometrical isomerism.
(C) [Co(NH\(_3\))\(_4\)Cl\(_2\)]\(^+\): This complex is of the type [MA\(_4\)B\(_2\)]. Similar to the first option, this complex can exist in \textit{cis} and \textit{trans} forms, depending on whether the two 'B' ligands (Cl) are adjacent (90\(^{\circ}\) apart) or opposite (180\(^{\circ}\) apart). Thus, it shows geometrical isomerism.
(D) [Co(NH\(_3\))\(_3\)(NO\(_2\))\(_3\)]: This complex is of the type [MA\(_3\)B\(_3\)]. Such complexes can exist in two geometrical forms: \textit{facial} (fac), where the three identical ligands occupy the corners of one triangular face of the octahedron, and \textit{meridional} (mer), where the three identical ligands lie in a plane that bisects the octahedron. Thus, it shows geometrical isomerism.
Step 3: Final Answer
Based on the analysis, the complex of the type [MA\(_5\)B], which is [Co(CN)\(_5\)(NC)]\(^{3-}\), does not show any geometrical isomerism.