In the given complexes, the heteroleptic complexes is/are:

A. \([Co(NH_3)_6]^{3+}\)

B. \([Cr(H_2O)_6]^{3+}\)

C. \([Ni(Cl_2)(PPh_3)_2]\)

D. \([Co(NH_3)_4Cl_2]^+\)

Choose the correct answer from the options given below:

C and D only

The correct answer is option 2. C and D only.

Heteroleptic complex: A coordination complex in which the central metal ion is bound to more than one type of ligand.

Analysis of options:

A. \([Co(NH_3)_6]^{3+}\): All six ligands are \(NH_3\), making it homoleptic, not heteroleptic.

B. \([Cr(H_2O)_6]^{3+}\): All six ligands are \(H_2O\), making it homoleptic, not heteroleptic.

C. \([Ni(Cl_2)(PPh_3)_2]\): This complex has two \(Cl\) ligands and two \(PPh_3\) ligands, satisfying the definition of heteroleptic.

D. \([Co(NH_3)_4Cl_2]^+\): This complex has four \(NH_3\) ligands and two \(Cl\) ligands, making it heteroleptic.

Therefore, the correct answer is 2. C and D only.

Key points to remember:

1. Heteroleptic complexes offer greater diversity in properties due to the different electronic and steric effects of various ligands.
2. They are often more common in biological systems, where metal ions need to interact with different molecules efficiently.