Which of the following species is not expected to be a ligand?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → NH4+
A ligand is an ion or molecule that binds to the central metal atom by donating a lone pair of electrons. So a species can act as a ligand only if it has at least one free lone pair on a donor atom.
In \( \ce{NH3} \), nitrogen has one lone pair. In NH4+, that lone pair has already been used to bind \( \ce{H^{+}} \), so nitrogen has no lone pair left (four N–H bonds, positive charge). \( \ce{NH4^{+}} \) therefore cannot act as a ligand.
Option (1) NO — A ligand. The nitrosyl ligand donates a lone pair from nitrogen, as in \( \ce{[Fe(H2O)5(NO)]^{2+}} \).
Option (2) NH4+ — Not a ligand. The lone pair on nitrogen is already used to bind H+.
Option (3) NH2NH2 — A ligand. Hydrazine has a lone pair on each nitrogen atom.
Option (4) CO — A ligand. The carbonyl ligand donates the lone pair on carbon, as in \( \ce{Ni(CO)4} \).