Which of the following cannot act as a ligand?

The correct answer is Option (3) → [H₃N–CH₂–CH₂–NH₃]²⁺

A ligand bonds to the metal by donating an electron pair, so it needs a donor atom that carries a lone pair.

Option (1) H₂N–CH₂–COO⁻ — Can act as a ligand. The N atom and the carboxylate O atoms carry lone pairs, so it can donate an electron pair to a metal.

Option (2) H₂N–NH₂ — Can act as a ligand. Each N atom has a lone pair to donate.

Option (3) [H₃N–CH₂–CH₂–NH₃]²⁺ — Cannot act as a ligand. This is ethane-1,2-diamine with both N atoms protonated: each lone pair is already used in an N–H bond, so no electron pair is left to donate.

Option (4) NH₃ — Can act as a ligand. A common unidentate ligand that donates the lone pair on N.

NCERT (2026-27 edition):
5.5.1 Valence Bond Theory – “… These hybridised orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.”