Which of the following is a `pi`-acid ligand?

Options

A. `NH_(3)`
B. CO
C. `F^(-)`
D. (en)

Answer

To determine which of the given ligands is a pi-acid ligand, we need to analyze each option based on the definition of pi-acid ligands. A pi-acid ligand is a Lewis acid that can accept electrons in its pi orbital.

### Step-by-Step Solution:

1. **Understanding Pi-Acid Ligands**:
- A pi-acid ligand is defined as a ligand that can accept electron density into its pi orbitals. This typically involves back-bonding where the metal donates electron density into the empty pi* (pi-star) orbitals of the ligand.

2. **Analyzing Each Option**:
- **Option A: NH3 (Ammonia)**:
- Ammonia has 10 electrons and does not have vacant pi orbitals. It acts as a sigma donor but does not accept electrons in a pi orbital. Therefore, NH3 is not a pi-acid ligand.

- **Option B: CO (Carbon Monoxide)**:
- Carbon monoxide has 15 electrons. It has a filled sigma orbital and vacant pi* orbitals. CO can accept electron density from the metal into its pi* orbitals through back-bonding. Hence, CO is a pi-acid ligand.

- **Option C: F- (Fluoride Ion)**:
- The fluoride ion has 10 electrons and is primarily a sigma donor. It does not have vacant pi orbitals to accept electrons, making it not a pi-acid ligand.

- **Option D: EN (Ethylene Diamine)**:
- Ethylene diamine has two nitrogen atoms, each with a lone pair that can donate electrons. However, it does not have vacant pi orbitals to accept electrons, so it is also not a pi-acid ligand.

3. **Conclusion**:
- After analyzing all the options, the only ligand that qualifies as a pi-acid ligand is **CO (Carbon Monoxide)**.

### Final Answer:
The correct answer is **Option B: CO (Carbon Monoxide)**.