Which of the following species is not expected to be a ligand?

Options

A. `O_(2)^(-)`
B. `NO^(+)`
C. `NH_(4)^(+)`
D. `CO`

Answer

To determine which of the given species is not expected to be a ligand, we need to analyze each species based on their ability to donate electron pairs to a central metal atom. Ligands can be neutral molecules, anions, or cations that have lone pairs of electrons available for donation.

### Step-by-Step Solution:

1. **Identify the Species**: The species given are O2-, NO+, NH4+, and CO.

2. **Analyze O2-**:
- O2- (oxide ion) has extra electrons, making it an anion.
- It has lone pairs of electrons that can be donated to a metal atom.
- **Conclusion**: O2- can act as a ligand.

3. **Analyze NO+**:
- NO+ (nitrosonium ion) is a cation.
- The nitrogen atom in NO+ has an unpaired electron, which can be donated to a metal atom.
- **Conclusion**: NO+ can act as a ligand.

4. **Analyze NH4+**:
- NH4+ (ammonium ion) has a positive charge and consists of nitrogen bonded to four hydrogen atoms.
- There are no lone pairs of electrons on the nitrogen atom in NH4+, meaning it cannot donate any electrons.
- **Conclusion**: NH4+ cannot act as a ligand.

5. **Analyze CO**:
- CO (carbon monoxide) is a neutral molecule.
- It has a lone pair of electrons on the carbon atom that can be donated to a metal atom.
- **Conclusion**: CO can act as a ligand.

### Final Answer:
Based on the analysis, the species that is not expected to be a ligand is **NH4+**.

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