Which of the following complexes is heteroleptic?

The correct answer is Option (2) → [Fe(NH₃)₄Cl₂]⁺

A heteroleptic complex has the metal bound to more than one kind of donor group. In [Fe(NH₃)₄Cl₂]⁺ the iron carries two kinds of ligand, NH₃ and Cl⁻.

Why the other options are incorrect

Option (1) [Cr(NH₃)₆]³⁺ — Incorrect. Only NH₃ is bound, so it is homoleptic.

Option (3) [Mn(CN)₆]⁴⁻ — Incorrect. Only CN⁻ is bound, so it is homoleptic.

Option (4) [Co(NH₃)₆]²⁺ — Incorrect. Only NH₃ is bound, so it is homoleptic.

NCERT (2026-27 edition):
5.2 Definitions of Some Important Terms Pertaining to Coordination Compounds – “… Complexes in which a metal is bound to more than one kind of donor groups, e.g., [Co(NH₃)₄Cl₂]⁺, are known as heteroleptic.”