Which of the following is a heteroleptic complex?

The correct answer is Option (3) → $[Co(NH_3)_4Cl_2]$

A heteroleptic complex contains more than one type of ligand attached to the central metal ion. A homoleptic complex contains only one type of ligand.

Option-wise Explanation

1. $[\text{Cr}(\text{NH}_3)_6]^{3+}$

All six ligands are $\text{NH}_3$ (ammine). Only one kind of ligand is present. This is a homoleptic complex.

2. $[\text{Fe}(\text{NH}_3)_6]^{3+}$

Again, six identical $\text{NH}_3$ ligands coordinate to iron. Since there is only one ligand type, it is also homoleptic.

3. $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]$

This complex contains two different ligands: $\text{NH}_3$ and $\text{Cl}^-$. Because more than one ligand type is present, this is a heteroleptic complex.

4. $[\text{Mn}(\text{CN})_6]^{4-}$

All six ligands are $\text{CN}^-$. Only one ligand type is involved, so this is homoleptic.