Identify the most stable complex species

The correct answer is Option (3) → \([Fe(C_2O_4)_3]^{3–}\)

When a di- or polydentate ligand binds a metal ion through two or more donor atoms at the same time, it forms a ring with the metal; such a ligand is called a chelate ligand. Chelate complexes are more stable than similar complexes of unidentate ligands. This extra stability is the chelate effect: to remove a chelating ligand, both of its bonds to the metal must break.

The oxalate ion, \( \ce{C2O4^{2-}} \), is a bidentate ligand: it binds through two oxygen atoms. In \( \ce{[Fe(C2O4)3]^{3-}} \) three oxalate ions form three five-membered chelate rings around \( \ce{Fe^{3+}} \). Its charge balance:

    \( \displaystyle x + 3(-2) = -3 \;\Rightarrow\; x = +3 \)

By the chelate effect, the oxalato complex [Fe(C₂O₄)₃]³⁻, the only chelate complex among the options, is the most stable of these species.

Why the other options are incorrect

• Option (1) \([Fe(CO)_5]\) — Incorrect. A carbonyl of iron in the zero oxidation state with five unidentate CO ligands; no chelate rings.

• Option (2) \([Fe(CN)_6]^{3–}\) — Incorrect. \( \ce{CN^{-}} \) is a strong field but unidentate ligand, so the complex has no chelate rings.

• Option (4) \([Fe(H_2O)_6]^{3+}\) — Incorrect. Water is a weak field, unidentate ligand, so this complex has no chelate rings either.

NCERT (2026-27 edition):
Definitions of Some Important Terms Pertaining to Coordination Compounds – …

Note: The same/similar question has been asked in different years by NTA as per the details given below: