The species, which is not a ligand is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(NO_2\)
A ligand is an ion or molecule that donates an electron pair to the central metal atom or ion. Nitrogen–oxygen species act as ligands as the nitrato ion, the nitrosyl group and the nitrite ion (nitrito-N / nitrito-O, an ambidentate ligand).
Option (1) \(NO_3^−\) — A ligand. The nitrato ligand binds through an oxygen atom.
Option (2) \(NO\) — A ligand. The neutral nitrosyl ligand binds through nitrogen, as in \( \ce{[Fe(CN)5NO]^2-} \).
Option (3) \(NO^+\) — A ligand. The nitrosonium ion also coordinates through nitrogen.
Option (4) \(NO_2\) — Not a ligand. The ligand of this family is the nitrite ion, \( \ce{NO2^-} \); the neutral \( \ce{NO2} \) molecule is not one of the usual ligands.