Which of these is least likely to act as Lewis base?

Options

A. CO
B. `F^(-)`
C. `BF_(3)`
D. `PF_(3)`

Explanation

To determine which of the given species is least likely to act as a Lewis base, we need to understand the definitions of Lewis acids and bases.

### Step-by-Step Solution:

1. **Understanding Lewis Acids and Bases**:
- Lewis acids are species that can accept an electron pair, typically because they have a vacant orbital or a positive charge.
- Lewis bases are species that can donate an electron pair, typically because they have a lone pair of electrons or a negative charge.

2. **Analyzing the Given Species**:
- We need to evaluate each species to see if they can act as a Lewis base.
- The species mentioned in the video are carbon monoxide (CO), fluoride ion (F⁻), phosphorus trifluoride (PF₃), and phosphorus pentafluoride (PF₅).

3. **Evaluating Each Species**:
- **Carbon Monoxide (CO)**: CO has a lone pair of electrons on the carbon atom, which can be donated. Therefore, CO can act as a Lewis base.
- **Fluoride Ion (F⁻)**: F⁻ has a negative charge and a lone pair of electrons, allowing it to donate electrons. Thus, F⁻ acts as a Lewis base.
- **Phosphorus Trifluoride (PF₃)**: PF₃ has a lone pair of electrons on the phosphorus atom, which can also be donated. Hence, PF₃ is a Lewis base.
- **Phosphorus Pentafluoride (PF₅)**: PF₅ does not have a lone pair of electrons available for donation. Instead, it has a full octet and can accept electron pairs due to the presence of vacant orbitals on phosphorus. Therefore, PF₅ acts as a Lewis acid.

4. **Conclusion**:
- Among the species analyzed, PF₅ is the least likely to act as a Lewis base because it does not have a lone pair of electrons to donate and primarily functions as a Lewis acid.

### Final Answer:
**PF₅ is least likely to act as a Lewis base.**