Which of the following is an ambidentate ligand?

The correct answer is Option (3) → SCN⁻

An ambidentate ligand has two different donor atoms but binds the metal through only one of them at a time. The thiocyanate ion can bind through sulphur (M–SCN) or through nitrogen (M–NCS).

Why the other options are incorrect

Option (1) NH₃ — Incorrect. It has only one donor atom, N, so it is a unidentate ligand.

Option (2) H₂O — Incorrect. It donates only through O, so it is also unidentate.

Option (4) H₂N–CH₂–CH₂–NH₂ — Incorrect. Ethane-1,2-diamine has two donor atoms of the same kind (both N) and uses both at the same time; it is a didentate (chelating) ligand.

NCERT (2026-27 edition):
5.2 Definitions of Some Important Terms Pertaining to Coordination Compounds – “… Ligand which has two different donor atoms and either of the two ligetes in the complex is called ambidentate ligand. Examples of such ligands are the NO₂⁻ and SCN⁻ ions.”