Which of the following is not a Lewis acid ?

Options

A. `BF_(3)`
B. `AlCl_(3)`
C. `FeCl_(3)`
D. `PH_(3)`

Explanation

To determine which of the following compounds is not a Lewis acid, we first need to understand the definition of a Lewis acid. A Lewis acid is defined as an electron pair acceptor, meaning it has the ability to accept an electron pair from a Lewis base, which is an electron pair donor.

Let's analyze the given options:

1. **BF3 (Boron Trifluoride)**:
- Boron has an incomplete octet and has vacant p-orbitals. It can accept an electron pair, thus it is a Lewis acid.

2. **AlCl3 (Aluminum Chloride)**:
- Aluminum in AlCl3 also has an incomplete octet and can accept an electron pair. Therefore, it is a Lewis acid.

3. **FeCl3 (Iron(III) Chloride)**:
- Iron in FeCl3 can also accept an electron pair due to the presence of vacant d-orbitals. Thus, it is a Lewis acid.

4. **PH3 (Phosphine)**:
- Phosphorus in PH3 has a complete octet and possesses lone pairs of electrons. Instead of accepting electron pairs, it tends to donate its lone pairs. Therefore, PH3 acts as a Lewis base, not a Lewis acid.

From this analysis, we can conclude that the compound that is not a Lewis acid is **PH3**.

### Final Answer:
The compound that is not a Lewis acid is **PH3** (option D).