Which of the following is not acid ligand?

Options

A. `CO`
B. `F^(-)`
C. `C_(2)H_(4)`
D. `NO^(+)`

Answer

To determine which of the following is not an acid ligand, we need to understand what an acid ligand is. An acid ligand is a ligand that has the ability to accept electron pairs from a metal center, typically through its π* (pi anti-bonding) orbitals.

### Step-by-Step Solution:

1. **Define Acid Ligands**:
- Acid ligands are those ligands that possess π* (pi anti-bonding) orbitals. These ligands can accept electron pairs from a metal center due to the availability of these orbitals.

2. **Examples of Acid Ligands**:
- Common examples of acid ligands include:
- **Carbon monoxide (CO)**: It has π* orbitals that can accept electrons from metals.
- **Ethylene (C2H4)**: It also has π* orbitals available for coordination with metals.
- **Nitronium ion (NO⁺)**: Similar to the above, it has π* orbitals that can interact with metal centers.

3. **Identify Non-Acid Ligands**:
- To find a ligand that is not an acid ligand, we need to look for one that does not have π* orbitals.
- **Fluorine (F)** is a good candidate. When a metal approaches fluorine, it does not have available π* orbitals to accept electrons because it is already a strong electron-withdrawing group and holds a negative charge.

4. **Conclusion**:
- Among the options provided, fluorine does not act as an acid ligand because it lacks the necessary π* orbitals to accept electrons from the metal.

5. **Final Answer**:
- The correct option is fluorine, as it is not an acid ligand.