Which of the following ligand does not as `pi-`acid ligand?

Options

A. `N_(2)`
B. `CO`
C. `C_(2)H_(4)`
D. `O_(2)^(2-)`

Explanation

To determine which ligand does not act as a pi-acid ligand, we need to understand the characteristics of pi-acid ligands. Pi-acid ligands are non-classical ligands that can donate an electron pair to the metal while simultaneously accepting electron density from the metal into their vacant orbitals. For a ligand to be classified as a pi-acid ligand, it must have pi bonds and vacant d-orbitals to accept electrons.

Let's analyze the given ligands:

1. **N₂ (Nitrogen gas)**:
- N₂ has a triple bond, which includes a pi bond. It can act as a pi-acid ligand because it can donate electrons and has vacant orbitals to accept electron density from the metal.

2. **CO₃²⁻ (Carbonate ion)**:
- The carbonate ion has resonance structures that include pi bonds. It can also act as a pi-acid ligand due to its ability to donate electrons and accept electron density.

3. **C₂H₄ (Ethene)**:
- Ethene has a double bond (which includes a pi bond). It can act as a pi-acid ligand as it can donate electrons and has vacant orbitals to accept electron density.

4. **O₂²⁻ (Peroxide ion)**:
- The peroxide ion has a sigma bond and does not possess pi bonds. Therefore, it cannot donate electrons in the same manner as the other ligands and does not have vacant d-orbitals to accept electron density. Thus, it does not act as a pi-acid ligand.

Based on this analysis, the ligand that does not act as a pi-acid ligand is **O₂²⁻ (Peroxide ion)**.

### Step-by-Step Solution:
1. **Identify the characteristics of pi-acid ligands**: They must have pi bonds and vacant d-orbitals to accept electrons.
2. **Analyze each ligand**:
- N₂: Has pi bonds, can act as a pi-acid ligand.
- CO₃²⁻: Has pi bonds, can act as a pi-acid ligand.
- C₂H₄: Has pi bonds, can act as a pi-acid ligand.
- O₂²⁻: Lacks pi bonds, cannot act as a pi-acid ligand.
3. **Conclusion**: The ligand that does not act as a pi-acid ligand is O₂²⁻.