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 among the following is an ambidentate ligand?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(SCN^-\)
An ambidentate ligand has two different donor atoms and can bind to the metal through either one of them, but through only one at a time. The thiocyanate ion is the standard example: it can coordinate through sulphur (M–SCN, thiocyanato-S) or through nitrogen (M–NCS, thiocyanato-N). The nitrite ion behaves in the same way, through N (nitrito-N) or O (nitrito-O).
\( \displaystyle \ce{M <- SCN} \quad \text{or} \quad \ce{M <- NCS} \)
Because the two modes of binding give different compounds, ambidentate ligands give rise to linkage isomerism. So \( \ce{SCN^-} \) is an ambidentate ligand.
Why the other options are incorrect
Option (2) \(NH_3\) — Incorrect. Ammonia has only one donor atom, nitrogen, with one lone pair, so it is a unidentate ligand (ammine).
Option (3) \(Cl^-\) — Incorrect. The chloride ion has a single donor atom and binds only through chlorine (chlorido), so it is unidentate.
Option (4) \(CO\) — Incorrect. Carbon monoxide binds to metals through the carbon atom (carbonyl ligand, as in \( \ce{[Ni(CO)4]} \)); it is treated as a unidentate ligand.
Note: In the original NTA paper, Option (3) was printed as CN−. The cyanide ion can bind to a metal through either carbon (M–CN) or nitrogen (M–NC), so it is also an ambidentate ligand, and NTA accepted both Option (1) and Option (3). So that students are not confused and the question has a single answer, Option (3) has been changed to Cl−, which binds only through chlorine.
NCERT (2026-27 edition):
Ligands – “… Ligand which has two different donor atoms and either of the two ligates in the complex is called ambidentate ligand. Examples of such ligands are the NO2– and SCN– ions. NO2– ion can coordinate either through nitrogen or through oxygen to a central metal atom/ion. Similarly, SCN– ion can coordinate through the sulphur or nitrogen atom.”