Which one of the following is an ambidentate ligand?

An ambidentate ligand is a type of ligand that possesses two different atoms capable of donating a lone pair to a metal centre, allowing it to bind to the metal through either of these donor atoms. This is distinct from a regular bidentate ligand, which has two donor atoms of the same type that coordinate simultaneously.

Let us examine each of the given options:

1. Ethylenediaminetetraacetate ion (EDTA) – EDTA is a hexadentate ligand that uses six oxygen atoms (from carboxylate groups) and two nitrogen atoms to chelate a metal. All of its donor atoms are of the same type in each binding site, so it is not ambidentate.

2. Oxalate – Oxalate (C₂O₄²⁻) coordinates a metal through its two oxygen atoms. Both donor atoms are identical, making oxalate a bidentate chelating ligand, not ambidentate.

3. Ethane-1,2-diamine – This molecule contains two nitrogen atoms that can simultaneously bind to a metal centre. Both donor atoms are nitrogen, so it is a bidentate ligand, not ambidentate.

4. Thiocyanate (SCN⁻) – Thiocyanate can bind to a metal either through the sulfur atom (forming a thiocyanato‑S complex) or through the nitrogen atom (forming a thiocyanato‑N complex). Because it offers two distinct donor atoms, it satisfies the definition of an ambidentate ligand.

Therefore, among the listed species, the ligand that can attach to a metal centre via two different atoms is thiocyanate.