Which will form the most stable complex?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C2O42−
The oxalate ion is a didentate ligand: it binds a metal ion through two oxygen atoms at the same time and forms a five-membered ring with the metal. Such ring-forming (chelating) ligands give chelate complexes, which are more stable than similar complexes containing unidentate ligands (the chelate effect).
Why the other options are incorrect
Option (1) NCS− — Incorrect. An ambidentate but unidentate ligand; it binds through N or S, only one atom at a time, so it forms no chelate ring.
Option (2) CN− — Incorrect. A strong-field ligand, but it is unidentate and binds through carbon only; it does not get the extra stability of chelation that oxalate provides.
Option (3) H2O — Incorrect. A weak, unidentate ligand; aqua complexes are among the least stable of these.
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.”
Definitions of Some Important Terms Pertaining to Coordination Compounds – “… When a ligand can bind through two donor atoms as in H2NCH2CH2NH2 (ethane-1,2-diamine) or C2O42− (oxalate), the ligand is said to be didentate and when several donor atoms are present in a single ligand as in N(CH2CH2NH2)3, the ligand is said to be polydentate.”