Question

The complex cation which has two isomers is :

• A. \(\left[ Co \left( H _2 O \right)_6\right]^{3+}\)
• B. \(\left[ Co \left( NH _3\right)_5 Cl \right]^{+}\)
• C. \(\left[ Co \left( NH _3\right)_5 NO _2\right]^{2+}\)
• D. \(\left[ Co \left( NH _3\right)_5 Cl \right]^{2+}\)

Hint:

Consider the different types of isomerism in coordination compounds, such as linkage isomerism, ionization isomerism, geometrical isomerism, and optical isomerism.

Answer 1

The Correct Option is C

Step 1: Analyze the Complexes for Isomerism
We are looking for a complex cation that exhibits two isomers. Let’s analyze each option:
\([ \text{Co}(\text{H}_2\text{O})_6 ]^{3+}\): 
This complex has only one possible structure, as all ligands are the same (\(\text{H}_2\text{O}\)). So, it does not have isomers.
\([ \text{Co}(\text{NH}_3)_5\text{Cl} ]^{2+}\): 
This complex can only exhibit ionization isomerism if another counter ion is present within the complex to exchange positions with the \(\text{Cl}\) ligand. With only one \(\text{Cl}\), there’s no possibility of isomerism here.
\([ \text{Co}(\text{NH}_3)_5\text{NO}_2 ]^{2+}\): 
This complex can exhibit linkage isomerism because the \(\text{NO}_2\) ligand can coordinate to the metal ion through either the nitrogen atom (nitro isomer) or the oxygen atom (nitrito isomer). Thus, it has two isomers.
\([ \text{Co}(\text{NH}_3)_5\text{Cl} ]^{+}\): 
Same explanation as (2), no isomerism is possible
Conclusion
The correct option is (3): \([ \text{Co}(\text{NH}_3)_5\text{NO}_2 ]^{2+}\), which exhibits two isomers (nitro and nitrito isomers).