Which of the following species is not expected to be a ligand
`overset(o+)(NO)`
`NH_(4)^(o+)`
(c ) `NH_(2)-NH_(3)^(o+)`
`(CO)` .

Explanation

To determine which of the following species is not expected to be a ligand, we need to analyze each option based on their ability to donate a lone pair of electrons to a central metal atom. A ligand is defined as a species that can donate electron pairs to form coordinate bonds with a metal atom.

### Step-by-Step Solution:

1. **Understanding Ligands**:
- A ligand must have a lone pair of electrons available for donation to a metal atom. If a species does not have a lone pair, it cannot act as a ligand.

2. **Analyzing Each Option**:
- **Option (a) \( \overset{o+}{NO} \)** (Nitrosyl cation):
- The nitrosyl cation has a nitrogen atom with a lone pair of electrons that can be donated. Therefore, \( \overset{o+}{NO} \) can act as a ligand.

- **Option (b) \( NH_4^{+} \)** (Ammonium ion):
- The ammonium ion is formed from ammonia (\( NH_3 \)) when it reacts with a proton (\( H^+ \)). In this process, the lone pair of electrons on nitrogen is used to bond with \( H^+ \), resulting in no lone pairs remaining for donation. Thus, \( NH_4^{+} \) cannot act as a ligand.

- **Option (c) \( NH_2-NH_3^{+} \)** (Hydrazinium ion):
- The hydrazinium ion has nitrogen atoms that can donate lone pairs. The nitrogen in \( NH_2 \) has a lone pair available for donation, making it a potential ligand.

- **Option (d) \( CO \)** (Carbon monoxide):
- Carbon monoxide is a well-known ligand that can donate a lone pair of electrons from the carbon atom to a metal center. Thus, \( CO \) can act as a ligand.

3. **Conclusion**:
- After analyzing all the options, we find that the species that is not expected to be a ligand is **Option (b) \( NH_4^{+} \)**, as it does not possess any lone pairs of electrons to donate.

### Final Answer:
The species that is not expected to be a ligand is **\( NH_4^{+} \)** (Option b).