Question

Which among the following is an ambidentate ligand?

• A. $\text{CN}^-$
• B. $\text{NH}_3$
• C. $\text{H}_2\text{O}$
• D. $\text{NO}_2^-$

Hint:

Key Exam Tip:
Common ambidentate ligands include $\text{NO}_2^-$ (nitro/nitrito), $\text{SCN}^-$ (thiocyanato/isothiocyanato), and $\text{CN}^-$ (though often treated as simple monodentate when C-bonded). Remember to check for multiple potential donor atoms.

Answer 1

The Correct Option is D

An ambidentate ligand is a ligand that can coordinate to a central metal atom or ion through two different donor atoms. In other words, it has more than one potential donor atom, and it can bind to the metal in different ways.
Let's examine each option:
(A) $\text{CN}^-$ (Cyanide ion): The cyanide ion can coordinate to a metal ion through either the carbon atom or the nitrogen atom. For example, it can form complexes like $[\text{Fe}(\text{CN})_6]^{3-}$ where it acts as a monodentate ligand through the carbon atom. It can also act as a bridging ligand with both C and N atoms coordinating to different metal ions. However, in the context of a single donor atom, it is typically considered a monodentate ligand, although the possibility of carbon or nitrogen coordination exists. But compared to other options, it's less of a classic example of an ambidentate ligand in simple coordination chemistry problems where it primarily binds through carbon.
(B) $\text{NH}_3$ (Ammonia): Ammonia is a monodentate ligand. The nitrogen atom has a lone pair of electrons and is the only atom that can coordinate to the metal.
(C) $\text{H}_2\text{O}$ (Water): Water is also a monodentate ligand. The oxygen atom has lone pairs of electrons and is the only atom that can coordinate to the metal.
(D) $\text{NO}_2^-$ (Nitrite ion): The nitrite ion ($\text{NO}_2^-$) can coordinate to a metal ion through either the nitrogen atom or one of the oxygen atoms.
• When it coordinates through the nitrogen atom, it is called a nitro ligand (e.g., $[\text{Co}(\text{NH}_3)_5(\text{NO}_2)]^{2+}$).
• When it coordinates through an oxygen atom, it is called a nitrito ligand (e.g., $[\text{Co}(\text{NH}_3)_5(\text{ONO})]^{2+}$). Since the nitrite ion can bind through two different types of atoms (N or O), it is classified as an ambidentate ligand.
Therefore, $\text{NO}_2^-$ is an ambidentate ligand.

Step 1: Define an ambidentate ligand. An ambidentate ligand is a ligand that possesses two different donor atoms through which it can bind to a central metal ion.

Step 2: Analyze the donor atoms in each given ligand.
• $\text{CN}^-$: Can donate electrons from either the carbon atom or the nitrogen atom.
• $\text{NH}_3$: The nitrogen atom is the donor atom.
• $\text{H}_2\text{O}$: The oxygen atom is the donor atom.
• $\text{NO}_2^-$: Can donate electrons from the nitrogen atom or one of the oxygen atoms.

Step 3: Identify the ligand that can coordinate through more than one type of atom. While $\text{CN}^-$ has two potential donor atoms (C and N), it typically coordinates through one at a time in simple complexes. However, the nitrite ion ($\text{NO}_2^-$) is a textbook example of an ambidentate ligand because it can coordinate through the nitrogen atom (nitro) or an oxygen atom (nitrito).

Step 4: Select the ambidentate ligand from the given options. Based on the analysis, $\text{NO}_2^-$ is the ambidentate ligand. Final Answer: \(\boxed{\text{NO}_2^-}\)