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

Options

A. (a) `NH_3`
B. (b) `CO`
C. (c) `F^(-)`
D. (d) Ethylene diamine

Explanation

To determine which of the following is a `pi`-acid ligand, we need to analyze the given ligands based on their ability to accept electrons in their pi-orbitals. Let's go through the options step by step.

### Step 1: Understand the Definition of Pi-Acid Ligands
A pi-acid ligand is a type of Lewis acid that can accept electron density into its pi-orbitals. This typically involves the presence of empty pi* (pi-star) orbitals that can accept electrons from a metal center.

### Step 2: Analyze Each Ligand

1. **NH3 (Ammonia)**
- NH3 has a lone pair of electrons on nitrogen and acts as a sigma donor.
- It does not have empty pi* orbitals to accept electrons, thus it is not a pi-acid ligand.

2. **CO (Carbon Monoxide)**
- CO has 15 electrons and can be represented in molecular orbital theory.
- It has a filled sigma bonding orbital and empty pi* orbitals.
- CO can donate electron density through its sigma bond and accept electron density through its pi* orbitals, making it a sigma donor and a pi-acid ligand.

3. **F- (Fluoride Ion)**
- F- has 10 electrons and primarily acts as a sigma donor.
- It does not have empty pi* orbitals to accept electrons, so it is not a pi-acid ligand.

4. **Ethylene Diamine (en)**
- Ethylene diamine has nitrogen atoms that can donate lone pairs.
- It primarily acts as a sigma donor and does not have empty pi* orbitals for pi-acceptance, thus it is not a pi-acid ligand.

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

### Final Answer
**C. CO (Carbon Monoxide)** is a pi-acid ligand.