Which of the following is not a neutral ligand ?

Options

A. `H_2O`
B. `NH_3`
C. `ONO`
D. `CO`

Explanation

To determine which of the given options is not a neutral ligand, we will analyze each option based on the definition of a neutral ligand.

### Step-by-Step Solution:

1. **Understand the Definition of a Ligand**:
- A ligand is defined as an ion or molecule that binds to a central atom or ion in a coordination compound.

2. **Define Neutral Ligands**:
- Neutral ligands are those that have no charge, meaning they are neither positively charged nor negatively charged.

3. **Analyze Each Option**:
- **Option A: H2O (Water)**
- Water is a neutral molecule and does not carry any charge. Therefore, it is a neutral ligand.

- **Option B: NH3 (Ammonia)**
- Ammonia is also a neutral molecule with no charge. Thus, it is a neutral ligand.

- **Option C: ONO (Nitrito)**
- The nitrito ligand (ONO) typically carries a negative charge. Therefore, it is not a neutral ligand.

- **Option D: CO (Carbon monoxide)**
- Carbon monoxide is a neutral molecule and does not carry any charge. Hence, it is a neutral ligand.

4. **Conclusion**:
- Based on the analysis, the ligand that is not neutral is **ONO** (Option C), as it has a negative charge.

### Final Answer:
**The ligand that is not neutral is ONO.**