Q.Assertion (A) : Linkage isomerism arises in coordination compounds because of ambidentate ligand. Reason (R) : Ambidentate ligand like $NO_2$ has two different donor atoms i.e., N and O. (A) Both Assertion (A) and Reason (R) are correct statements, and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are correct statements, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is correct, but Reason (R) is incorrect statement. (D) Assertion (A) is incorrect, but Reason (R) is correct statement.

Concept: Linkage isomerism — a type of structural isomerism where the same ligand bonds through different donor atoms.

Reasoning:

1. Assertion (A) is correct: Linkage isomerism occurs precisely because ambidentate ligands (like $\text{NO}_2^-$, $\text{SCN}^-$) can coordinate through either of two different atoms.
2. Reason (R) is also correct: The nitrite ion $\text{NO}_2^-$ can bind through nitrogen (forming nitro complexes) or through oxygen (forming nitrito complexes). …

Why this question tests a core idea

Coordination compounds can show isomerism in several ways. Linkage isomerism is a special type where the same ligand attaches to the metal through different atoms, producing distinct compounds. The key enabler is an ambidentate ligand — a ligand that has two (or more) different atoms capable of donating a lone pair to the metal.

The classic example is the nitrite ion, $\ce{NO2-}$. It can bind through nitrogen (forming a nitro complex) or through oxygen (forming a nitrito complex). These are not just different bonding modes — they are different isomers with distinct properties.

Step-by-step reasoning

1. What does the Assertion claim?

   "Linkage isomerism arises in coordination compounds because of ambidentate ligand."

   This is factually correct. Without a ligand that can coordinate through two different donor atoms, linkage isomerism cannot occur. Every known case of linkage isomerism involves an ambidentate ligand.

2. What does the Reason state?

   "Ambidentate ligand like $\ce{NO2}$ has two different donor atoms i.e., N and O."

   This is also correct. The nitrite ion is ambidentate: the nitrogen atom and either oxygen atom can donate a lone pair. (Note: the oxygen atoms are equivalent by resonance, but the two binding sites — N vs O — are chemically distinct.)

3. Is the Reason the correct explanation for the Assertion?

   Yes — and this is the crux. The reason directly explains why ambidentate ligands cause linkage isomerism: because they offer two different donor atoms, the metal can choose which one to bond to, leading to isomers that differ in the linkage atom.

   If a ligand had only one donor atom (like $\ce{NH3}$), no linkage isomerism would be possible. So the existence of two different donor atoms is the very mechanism that makes linkage isomerism possible. …