Pick out the incorrect statement

Options

A. `PH_(4)^(+)` ion is tetrahedral like the `NH_(4)^(+)` ion and is obained when `PH_(3)` is bonded to proton
B. `PH_(4)I` is one of the most stable salts containing the phosphonium ion. It is also more stable than ammonium salts
C. `PH_(4)I` is decomposed by water to form `PH_(3)`
D. `PH_(3)` converts silver salts in solution to silver phosphide, which subsequently reacts to give free metal

Answer

To solve the question of identifying the incorrect statement regarding the phosphonium ion (PH4+), we will analyze each statement provided in the options step by step.

### Step-by-Step Solution:

1. **Analyze Statement 1**:
- The statement claims that PH4+ is tetrahedral and is formed when PH3 bonds to a proton (H+).
- **Explanation**: PH3 has a trigonal pyramidal shape due to the presence of a lone pair on phosphorus. When it bonds with a proton, it loses one electron and forms PH4+, which indeed has a tetrahedral geometry because it has no lone pairs and four bonding pairs.
- **Conclusion**: This statement is correct.

2. **Analyze Statement 2**:
- The statement claims that PH4I is one of the most stable salts containing the phosphonium ion and is more stable than ammonium salts.
- **Explanation**: PH4I is actually less thermally stable than ammonium salts. This is because phosphine (PH3) is less basic than ammonia (NH3), which means it does not readily accept protons. Therefore, PH4I decomposes at a lower temperature (around 60°C), indicating it is less stable than ammonium salts.
- **Conclusion**: This statement is incorrect.

3. **Analyze Statement 3**:
- The statement claims that PH4I decomposes in water to form PH3.
- **Explanation**: When PH4I reacts with water, it does indeed decompose to form PH3, H3O+, and I-. This reaction is consistent with the known chemistry of phosphonium salts.
- **Conclusion**: This statement is correct.

4. **Analyze Statement 4**:
- The statement claims that PH3 converts silver salts in solution to silver phosphide, which subsequently reacts to give free metal.
- **Explanation**: PH3 can react with silver ions (Ag+) to form silver phosphide (Ag3P). This reaction is known and leads to the formation of free silver metal.
- **Conclusion**: This statement is correct.

### Final Conclusion:
The only incorrect statement among the options is **Statement 2**, which claims that PH4I is more stable than ammonium salts.