Which of the following is not a Chelating ligand?

thiosulphato

The correct answer is Option (1) → thiosulphato

A chelating ligand must be polydentate, meaning it should have two or more donor atoms that can bind to the same metal ion and form a ring structure (chelate ring). If a ligand can attach through only one donor atom at a time, it is monodentate and cannot chelate.

Option-wise Detailed Explanation

1. Thiosulphate (S₂O₃²⁻) Thiosulphate can bind to a metal ion through either sulfur or oxygen, but typically only one donor atom coordinates at a time. It behaves mainly as a monodentate or ambidentate ligand. Since it does not form stable chelate rings with one metal center, it is not a chelating ligand.

2. Ethane-1,2-diamine (en) This ligand has two nitrogen atoms, each with a lone pair. Both can coordinate simultaneously to the same metal ion. It forms a five-membered chelate ring, so it is a classic bidentate chelating ligand.

3. Glycinato Glycinato (from glycine) contains both an amine nitrogen and a carboxylate oxygen. These two donor atoms can bind to the same metal ion. Thus, it acts as a bidentate ligand and forms a chelate ring.

4. Oxalato (C₂O₄²⁻) Oxalate has two negatively charged oxygen atoms positioned to bind to a metal ion at two sites. It forms a stable five-membered chelate ring, making it a strong chelating ligand