Which of the following ligand is not `pi`-acceptor ligand ?

Options

A. `pi-C_(3)H_(5)^(-)`
B. `pi-C_(5)H_(5)^(-)`
C.
D. `PF_(3)`

Explanation

To determine which of the following ligands is not a pi-acceptor ligand, we need to analyze each option based on their ability to participate in pi backbonding.

### Step-by-Step Solution:

1. **Understanding Ligands**:
- Ligands can be classified into two main types: classical and non-classical (or pi-acceptor) ligands.
- Classical ligands donate electrons to the metal center only through sigma bonds, while non-classical ligands can donate through both sigma and pi bonds.

2. **Analyzing the Ligands**:
- **Option A: π-C3H5⁻ (Cyclopropenyl anion)**:
- This ligand can donate electrons through its pi bond due to the presence of a double bond.
- It can also accept electrons back from the metal into its pi* (antibonding) orbital.
- Therefore, this is a pi-acceptor ligand.

- **Option B: π-C5H5⁻ (Cyclopentadienyl anion)**:
- Similar to the previous ligand, cyclopentadienyl has pi bonds and can donate electrons through these pi bonds.
- It also has vacant pi* orbitals available for backbonding with the metal.
- Hence, this is also a pi-acceptor ligand.

- **Option C: C3H5⁻ (Cyclopropane anion)**:
- Cyclopropane does not have any pi bonds; it is a saturated hydrocarbon.
- It can only donate electrons through sigma bonds and does not have vacant pi* orbitals for backbonding.
- Therefore, this is NOT a pi-acceptor ligand.

- **Option D: PF3 (Phosphorus trifluoride)**:
- In PF3, phosphorus has a vacant d-orbital that can accept electrons.
- The ligand can participate in backbonding, making it a pi-acceptor ligand.

3. **Conclusion**:
- From the analysis, the ligand that is **not** a pi-acceptor ligand is **C3H5⁻ (Cyclopropane anion)**.

### Final Answer:
The ligand that is not a pi-acceptor ligand is **C3H5⁻ (Cyclopropane anion)**.

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