Which of the following complexes is/are heteroleptic?

(a) ${\left[Co{\left(N{H}_{3}\right)}_{6}\right]}^{3+}$
(b) ${\left[\mathrm{Fe}{\left({\mathrm{NH}}_{3}\right)}_{4}{\mathrm{Cl}}_{2}\right]}^{+}$
(c) ${\left[\mathrm{Mn}{\left(\mathrm{CN}\right)}_{6}\right]}^{4-}$
(d) $\left[\mathrm{Co}{\left({\mathrm{NH}}_{3}\right)}_{4}{\mathrm{Cl}}_{2}\right]$

Choose the correct option
1. (a, b)
2. (b, c)
3. (c, d)
4. (b, d)

Heteroleptic complexes Coordination complexes that contain more than one type of ligands are known as heteroleptic complexes.

eg, ${\left[\mathrm{Fe}{\left({\mathrm{NH}}_{3}\right)}_{4}{\mathrm{Cl}}_{2}\right]}^{+}$ contain NH, and Cl as a ligand is a heteroleptic complex. Similarly, $\left[\mathrm{Co}{\left({\mathrm{NH}}_{3}\right)}_{4}{\mathrm{Cl}}_{2}\right]$ contain ${NH}_{3}$ and Cl as ligand is also a heteroleptic complex

Hence, optons (b) and (d) are correct choices.