Which of the following is least likely to behave as Lew is acid?

Options

A. `OH^(-)`
B. `H_(2)O`
C. `NH_(3)`
D. ` BF_(3)`

Explanation

To determine which of the given compounds is least likely to behave as a Lewis acid, we need to understand the definition of a Lewis acid. A Lewis acid is a substance that can accept an electron pair. Typically, species that have incomplete octets or are electron-deficient are more likely to act as Lewis acids.

Let's analyze the options step by step:

1. **OH⁻ (Hydroxide Ion)**:
- The hydroxide ion has 8 electrons in its valence shell (2 from the oxygen-hydrogen bond and 6 from the lone pairs on oxygen).
- Since its octet is complete, it does not have the tendency to accept additional electron pairs.
- **Conclusion**: OH⁻ is not likely to behave as a Lewis acid.

2. **H₂O (Water)**:
- Water also has a complete octet with 8 electrons (2 from each hydrogen-oxygen bond and 4 from the lone pairs on oxygen).
- Similar to OH⁻, it does not have the capacity to accept more electrons.
- **Conclusion**: H₂O is not likely to behave as a Lewis acid.

3. **NH₃ (Ammonia)**:
- Ammonia has 8 electrons in total (2 from each N-H bond and 2 from the lone pair on nitrogen).
- Its octet is complete, so it is also not inclined to accept additional electron pairs.
- **Conclusion**: NH₃ is not likely to behave as a Lewis acid.

4. **BF₃ (Boron Trifluoride)**:
- Boron trifluoride has only 6 electrons in its valence shell (2 from each B-F bond).
- Since boron has an incomplete octet, it is electron-deficient and can accept an electron pair to complete its octet.
- **Conclusion**: BF₃ is likely to behave as a Lewis acid.

After analyzing all the options, we can conclude that the species least likely to behave as a Lewis acid is **OH⁻ (Hydroxide Ion)**, as it has a complete octet and does not have the tendency to accept electron pairs.

### Summary:
- **Least likely to behave as a Lewis acid**: OH⁻ (Hydroxide Ion)