Ambidentate ligands like `NO_(2)^(-) and SCN^(-)` are :

Options

A. unidentate
B. didentate
C. polydentate
D. has variable denticity

Answer

To answer the question regarding ambidentate ligands like `NO2^(-)` and `SCN^(-)`, we need to understand what ambidentate ligands are and how they function.

### Step-by-Step Solution:

1. **Definition of Ambidentate Ligands**: Ambidentate ligands are ligands that can coordinate to a metal ion through two different atoms. However, they can only bind through one donor atom at a time.

2. **Example of `NO2^(-)`**: The nitrite ion (`NO2^(-)`) can act as an ambidentate ligand because it has two potential donor atoms: nitrogen (N) and oxygen (O).
- When it coordinates through the nitrogen atom, it forms a different complex than when it coordinates through the oxygen atom.

3. **Example of `SCN^(-)`**: The thiocyanate ion (`SCN^(-)`) is another ambidentate ligand. It can coordinate through the sulfur atom (S) or the nitrogen atom (N).
- Similar to `NO2^(-)`, it can form different complexes depending on which atom is donating the electron pair to the metal.

4. **Coordination Behavior**: In both cases, the ligand can only coordinate through one atom at a time, which is a key characteristic of ambidentate ligands. This means that although they have multiple donor atoms, they cannot form bonds with the metal ion simultaneously through both donor atoms.

5. **Conclusion**: Therefore, ambidentate ligands like `NO2^(-)` and `SCN^(-)` are defined by their ability to bind through different atoms (N or O in the case of `NO2^(-)` and S or N in the case of `SCN^(-)`), but they do so one at a time.

### Final Answer:
Ambidentate ligands like `NO2^(-)` and `SCN^(-)` are ligands that can coordinate to a metal ion through two different donor atoms (N and O for `NO2^(-)`; S and N for `SCN^(-)`), but they can only bind through one donor atom at a time.