Which of the following is not a polydentate ligand?

Options

A. Oxalate ion
B. Ethylenediamine
C. Thiocyanato
D. EDTA

Answer

To determine which of the following is not a polydentate ligand, we need to first understand what a polydentate ligand is. A polydentate ligand is a ligand that can bind to a metal ion at multiple coordination sites. This means it has more than one donor atom that can attach to the metal ion.

### Step-by-Step Solution:

1. **Identify the Ligands**:
- List the ligands provided in the question (though not explicitly mentioned here, we will assume common examples like oxalate, ethylenediamine, water, etc.).

2. **Define Polydentate Ligands**:
- A ligand is considered polydentate if it has two or more donor atoms that can coordinate to a metal ion. For example, oxalate (C2O4^2-) has two carboxylate groups and can bind through both oxygen atoms, making it a bidentate ligand.

3. **Analyze Each Ligand**:
- **Oxalate ion (C2O4^2-)**: This is a bidentate ligand as it can bind through two oxygen atoms.
- **Ethylenediamine (en)**: This is also a bidentate ligand as it has two nitrogen atoms that can coordinate to a metal ion.
- **Water (H2O)**: This is a monodentate ligand as it has only one oxygen atom that can coordinate to a metal ion.
- **Acetate ion (CH3COO-)**: This is also a monodentate ligand as it can bind through only one oxygen atom.

4. **Conclusion**:
- From the analysis, we can conclude that the ligand that is not a polydentate ligand is **water (H2O)** or any other monodentate ligand listed in the options.

### Final Answer:
The ligand that is not a polydentate ligand is **water (H2O)**.