Question

Which one of the following compounds does not show geometrical isomerism?

• A. \(\text{MnO}_4^-\)
• B. \([\text{Fe}(\text{NH}_3)_2(\text{CN})_4]^-\)
• C. \([\text{CrCl}_2(\text{ox})_2]^{3-}\)
• D. \([\text{CoCl}_2(\text{en})_2]\)

Hint:

Tetrahedral complexes (\(\text{CN} = 4\)) NEVER show geometrical isomerism because all four positions are equivalent and adjacent to one another.

Answer 1

The Correct Option is A

Concept:
Geometrical isomerism (cis-trans isomerism) occurs in coordination compounds when ligands can be arranged in spatially distinct positions around the central metal atom. 
Tetrahedral complexes do not show geometrical isomerism because all four ligand positions are equivalent. 

Step 1: Geometry and Symmetry of \(\text{MnO}_4^-\): 
The permanganate ion, \(\text{MnO}_4^-\), has a regular tetrahedral geometry with four equivalent oxygen atoms surrounding the central manganese atom. 
In a tetrahedral arrangement, all four positions are equivalent, so distinct cis and trans arrangements are not possible. 
Therefore, \(\text{MnO}_4^-\) does not exhibit geometrical isomerism. 

Step 2: Assessing \([\text{Fe}(\text{NH}_3)_2(\text{CN})_4]^-\): 
This is a six-coordinate octahedral complex of the general type \([\text{MA}_2\text{B}_4]\). 
It can exhibit geometrical isomerism. 
In the cis-isomer, the two \(\text{NH}_3\) ligands occupy adjacent positions at \(90^\circ\). 
In the trans-isomer, the two \(\text{NH}_3\) ligands occupy opposite positions at \(180^\circ\). 

Step 3: Assessing \([\text{CrCl}_2(\text{ox})_2]^{3-}\) and \([\text{CoCl}_2(\text{en})_2]\): 
Both are octahedral complexes containing two monodentate chloro ligands and two bidentate ligands. 
They can show cis-trans geometrical isomerism depending on the relative positions of the two \(\text{Cl}^-\) ligands. 
When the two chloro ligands are adjacent, the complex is cis, while when they are opposite, the complex is trans. 

Final Answer: 
Hence, \(\text{MnO}_4^-\) does not show geometrical isomerism. 
\[ \boxed{\text{Option (A)}} \]