Read the Passage carefully and answer the questions.

When a ligand is bound to a metal ion through a single donor atom, it is said to be a unidentate ligand but when it bind through two donor atoms, then it is said to be didentate ligand and when several donor atoms are present in a single ligand, it is said to be a polydentate ligand. When a di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it is said to be a chelate ligand. The number of such ligating groups is called the denticity of the ligand. ligand which has two different donor atoms and either of the two ligetes in the complex is called ambidentate ligands. The number of ligand donor atoms to which the metal is directly bonded, is called coordination number of the metal ion in the complex. Complexes in which a metal is bound to only one kind of donor groups are known as homoleptic and in which a metal is bound to more than one kind of donor groups are known as heteroleptic complexes.

Which of the following molecule/ion is not expected to behave as ligand?

${NH_4}^+$

The correct answer is Option (2) → ${NH_4}^+$

The species not expected to behave as a ligand is: NH₄⁺

Reason:

• A ligand must have at least one lone pair of electrons to donate to a metal ion.
• NH₄⁺ (ammonium ion) has no lone pair on nitrogen (it is fully protonated), so it cannot donate electrons.
• NO, CN⁻, and I⁻ all possess lone pairs and can act as ligands.