Which one of the following is NOT a ligand ?

Options

A. `PH_(3)`
B. `NO^(+)`
C. `Br^(-)`
D. `BF_(3)`

Answer

To determine which of the following is NOT a ligand, we need to understand what a ligand is. A ligand is a species that can donate a pair of electrons to a metal ion to form a coordinate bond. Ligands can be classified as either neutral molecules or anions that have lone pairs of electrons available for donation.

Let's analyze the given options:

1. **PH3 (Phosphine)**:
- Phosphine has a phosphorus atom with a lone pair of electrons. It can donate this lone pair to a metal ion, thus acting as a ligand.

2. **NO⁺ (Nitrosonium ion)**:
- The nitrosonium ion has a nitrogen atom that can donate electrons. Therefore, it can also act as a ligand.

3. **Br⁻ (Bromide ion)**:
- The bromide ion has a lone pair of electrons and can donate these electrons to a metal ion, making it a ligand.

4. **BF3 (Boron trifluoride)**:
- Boron trifluoride is an electron-deficient compound. Boron has only six electrons in its valence shell and does not have a lone pair to donate. Therefore, BF3 cannot act as a ligand because it cannot donate electrons.

Based on this analysis, the answer to the question "Which one of the following is NOT a ligand?" is **BF3**.

### Summary of Steps:
1. Define what a ligand is.
2. Analyze each option to determine if it can donate electrons.
3. Conclude that BF3 is the only compound that cannot act as a ligand.