Which of the following is not a Chelating ligand?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → thiosulphato
A chelating ligand is a di- or polydentate ligand that binds a single metal ion through two or more donor atoms at the same time, forming a ring with the metal. Such ring-containing complexes, chelate complexes, are more stable than similar complexes of unidentate ligands.
The thiosulphato ligand, \( \ce{S2O3^2-} \), normally binds the metal through one donor atom only (the sulphur atom, or in some complexes an oxygen atom). It is a unidentate (ambidentate) ligand, as in the complex formed in photographic fixing:
\( \displaystyle \ce{AgBr + 2S2O3^2- -> [Ag(S2O3)2]^3- + Br-} \)
Option (1) thiosulphato — Not a chelating ligand (binds through one donor atom). Thiosulphato binds the metal through a single donor atom, so it closes no ring.
Option (2) ethane-1, 2-diamine — A chelating ligand. \( \ce{H2NCH2CH2NH2} \) (en) binds through both nitrogen atoms and forms a five-membered chelate ring, as in \( \ce{[Co(en)3]^3+} \).
Option (3) glycinato — A chelating ligand. The glycinate ion, \( \ce{H2NCH2COO-} \), binds through the nitrogen atom and one carboxylate oxygen atom, forming a five-membered ring.
Option (4) oxalato — A chelating ligand. \( \ce{C2O4^2-} \) binds through two oxygen atoms, one from each carboxylate group, as in \( \ce{[Fe(C2O4)3]^3-} \).
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.”