Identify the most stable complex.

The correct answer is Option (3) → [Co(en)₃]Cl₃

All four are Co(III) complexes:

    \( \ce{[Co(NH3)6]^3+} \): x = +3
    \( \ce{[CoCl2(en)2]+} \): x − 2 = +1, x = +3
    \( \ce{[Co(en)3]^3+} \): x = +3
    \( \ce{[CoCl4(en)]-} \): x − 4 = −1, x = +3

So the deciding factor is the chelate effect. Ethane-1,2-diamine (en) is a didentate ligand that binds through both N atoms and forms a five-membered ring with the metal. Chelate complexes tend to be more stable than similar complexes with unidentate ligands, and the more chelate rings, the greater the stability. \( \ce{[Co(en)3]^3+} \) has three chelate rings and six N donors, so it is the most stable.

Why the other options are incorrect

• Option (1) [Co(NH₃)₆]Cl₃ — Incorrect. Six N donors, but all unidentate \( \ce{NH3} \); no chelate ring.

• Option (2) [CoCl₂(en)₂]Br — Incorrect. Only two chelate rings, and two weak-field \( \ce{Cl^{-}} \) ligands.

• Option (4) K[CoCl₄(en)] — Incorrect. Only one chelate ring and four \( \ce{Cl^{-}} \) ligands; the least stable of the chelates.

NCERT (2026-27 edition):
Ligands – “… When a di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it is said to be a chelate ligand. The number of such ligating groups is called the denticity of the ligand. Such complexes, called chelate complexes tend to be more stable than similar complexes containing unidentate ligands.”