Which of the following cannot show linkage isomerism?

The correct answer is Option (4) → H₂O

Linkage isomerism needs an ambidentate ligand: a ligand with two different donor atoms, either of which can bind the metal. Water has only one donor atom, oxygen (the H atoms carry no lone pair), so it can bind in only one way.

Option (1) NO₂⁻ — Can show linkage isomerism. It binds through nitrogen (nitro, –NO₂) or through oxygen (nitrito, –ONO).

Option (2) CN⁻ — Can show linkage isomerism. In \( \ce{CN^-} \) both carbon and nitrogen carry a lone pair, so it can bind as M–CN or M–NC.

Option (3) SCN⁻ — Can show linkage isomerism. It binds through sulphur (M–SCN) or through nitrogen (M–NCS).

Option (4) H₂O — Cannot show linkage isomerism. It is a unidentate ligand with a single donor atom, oxygen.

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.”

5.4.3 Linkage Isomerism – “Linkage isomerism arises in a coordination compound containing ambidentate ligand.”