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

Options

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

Explanation

To determine which of the given species is not expected to be a ligand, we need to analyze each option based on their ability to donate electron pairs to a metal center. Ligands are defined as species that can donate at least one pair of electrons to form a coordinate bond with a metal ion.

### Step-by-step Solution:

1. **Identify the Species**: The options provided are:
- NO (Nitric Oxide)
- Ammonium ion (NH4+)
- NH2CH2CH2NH2 (Ethylenediamine)
- Carbonyl (CO)

2. **Analyze NO (Nitric Oxide)**:
- The structure of NO shows that nitrogen has a lone pair of electrons.
- This lone pair can be donated to a metal, allowing NO to act as a ligand.
- **Conclusion**: NO can act as a ligand.

3. **Analyze Ammonium Ion (NH4+)**:
- The ammonium ion has a tetrahedral structure with 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**: Ammonium ion cannot act as a ligand.

4. **Analyze NH2CH2CH2NH2 (Ethylenediamine)**:
- Ethylenediamine contains two nitrogen atoms, each with a lone pair of electrons.
- These lone pairs can be donated to a metal, allowing ethylenediamine to act as a bidentate ligand.
- **Conclusion**: Ethylenediamine can act as a ligand.

5. **Analyze Carbonyl (CO)**:
- The carbonyl group has a carbon atom double-bonded to an oxygen atom, with oxygen possessing lone pairs of electrons.
- The lone pair on oxygen can be donated to a metal, allowing carbonyl to act as a ligand.
- **Conclusion**: Carbonyl can act as a ligand.

6. **Final Conclusion**:
- Among the four species analyzed, the only one that cannot act as a ligand is the ammonium ion (NH4+), as it lacks any lone pairs to donate.

### Answer:
The species that is not expected to be a ligand is **Ammonium ion (NH4+)**.

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