Assertion (A): \( \ce{[Co(NH3)6]^3+} \) is a homoleptic complex.
Reason (R): The coordination number of cobalt in \( \ce{[Co(NH3)6]^3+} \) is 6.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
A complex is homoleptic when the metal is bound to only one kind of donor group, and heteroleptic when it is bound to more than one kind. The coordination number is the number of ligand donor atoms bonded to the metal.
Assertion (A): \( \ce{[Co(NH3)6]^3+} \) is a homoleptic complex. — True. All six ligands are NH3, so cobalt is bound to one kind of donor group only.
Reason (R): The coordination number of cobalt in \( \ce{[Co(NH3)6]^3+} \) is 6. — True. Six unidentate NH3 molecules each bond through one nitrogen atom, so Co is bonded to six donor atoms.
Why R does not explain A?
The coordination number counts how many donor atoms are bonded, not what kind they are; \( \ce{[Co(NH3)4Cl2]^+} \) also has coordination number 6 but is heteroleptic.
The correct reason for A is that cobalt is bound to only one kind of donor group, NH3.
NCERT (2026-27 edition):
5.2 (h) Homoleptic and heteroleptic complexes – “Complexes in which a metal is bound to only one kind of donor groups, e.g., [Co(NH3)6]3+, are known as homoleptic. Complexes in which a metal is bound to more than one kind of donor groups, e.g., [Co(NH3)4Cl2]+, are known as heteroleptic.”
5.2 (d) Coordination number – “The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.”