Which of the following complexes is heteroleptic?

[Fe(NH₃)₄Cl₂]⁺

The answer is (2) \([Fe(NH_3)_4Cl_2]^+\).

A heteroleptic complex is a complex in which the metal ion is bound to more than one type of ligand. The other complexes in the answer choices are homoleptic, meaning that they only have one type of ligand.

In \([Fe(NH_3)_4Cl_2]^+\), the metal ion is bound to four ammonia ligands and two chloride ligands. This makes it a heteroleptic complex.

The other complexes in the answer choices are homoleptic. \([Cr(NH_3)_6]^{3+}\) is a complex in which the metal ion is bound to six ammonia ligands. \([Mn(CN)_6]^{4–}\) is a complex in which the metal ion is bound to six cyanide ligands. \([Co(NH_3)_6]^{+2}\) is a complex in which the metal ion is bound to six ammonia ligands.