The ligand which is not chelating is :

Options

A. EDTA
B. `C_2O_4^(2-)`
C. `CH_3COO^(-)`
D. `en`.

Explanation

To determine which ligand is not chelating, we need to understand the concept of chelation and the types of ligands.

### Step-by-Step Solution:

1. **Define Chelating Ligands**:
- Chelating ligands are polydentate ligands that can bind to a central metal atom at multiple points, forming a ring structure. This property is known as chelation.

2. **Identify the Ligands Given**:
- The ligands mentioned in the question are:
- EDTA (Ethylenediaminetetraacetic acid)
- C₂O₄²⁻ (Oxalate ion)
- CH₃C(=O)O⁻ (Acetate ion)
- Ethylene diamine (en)

3. **Classify Each Ligand**:
- **EDTA**: This is a hexadentate ligand, meaning it has six donor atoms and can form multiple bonds with a metal ion, thus exhibiting chelation.
- **C₂O₄²⁻ (Oxalate)**: This is a bidentate ligand, having two donor atoms that can bind to a metal ion, also exhibiting chelation.
- **CH₃C(=O)O⁻ (Acetate)**: This is a monodentate ligand, meaning it has only one donor atom and can only bind to a metal ion at one point. Therefore, it does not exhibit chelation.
- **Ethylene diamine (en)**: This is a bidentate ligand, having two donor atoms, and can also exhibit chelation.

4. **Conclusion**:
- Among the ligands listed, the acetate ion (CH₃C(=O)O⁻) is the only ligand that is not chelating because it is monodentate and can only form one bond with a metal ion.

### Final Answer:
The ligand which is not chelating is **CH₃C(=O)O⁻ (Acetate ion)**.