Which of the following gives the correct order of stability constant in the solution?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \([Cr(NH_3)_6]^{3+}\lt[Cr(en)_3]^{3+}\)
A ligand that binds the metal through two or more donor atoms forms a ring (a chelate). Chelate complexes are more stable than similar complexes of unidentate ligands (the chelate effect). Ethane-1,2-diamine (en) is a bidentate ligand with two N donors, while \( \ce{NH3} \) is unidentate with one N donor. So \( \ce{[Cr(en)3]^{3+}} \) has the higher stability constant than \( \ce{[Cr(NH3)6]^{3+}} \), and the order in option (1) is correct.
Why the other options are incorrect
Option (2) \([Ni(CO)_4]\lt[NiCl_4]^{2-}\) — Incorrect. CO is a strong field ligand that forms both a σ bond and a π back bond (synergic bonding) with nickel, so \( \ce{[Ni(CO)4]} \) is the more stable; the order is reversed.
Option (3) \([Sn(en)_3]^{3+}\lt[Sn(H_2O)_6]^{3+}\) — Incorrect. By the chelate effect the en complex is the more stable, so the order is reversed.
Option (4) \([Cr(NH_3)_6]^{3+}\lt[Cr(H_2O)_6]^{3+}\) — Incorrect. \( \ce{NH3} \) is a stronger ligand than \( \ce{H2O} \) (it stands higher in the spectrochemical series and replaces water from the aqua ion), so the ammine complex is the more stable.
NCERT (2026-27 edition):
Definitions of Some Important Terms Pertaining to Coordination Compounds – “… 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.”