Which of the following do not exist ?

Options

A. `NCl_(5)`
B. `PH_(5)`
C. `[BCl_(6)]^(3-)`
D. All of these

Answer

To determine which of the given compounds do not exist, we will analyze each compound one by one. The options provided are NCl5, Ph5, BCl6^3-, and the option that states all of these do not exist.

### Step 1: Analyze NCl5
- **Covalency and Hybridization**: Nitrogen typically forms three bonds (covalency of 3) due to its electronic configuration. To form NCl5, nitrogen would need to expand its covalency to 5.
- **Vacant d-Orbitals**: However, nitrogen does not have vacant d-orbitals available to accommodate additional bonding. Therefore, NCl5 cannot exist.

### Step 2: Analyze Ph5
- **Covalency and Hybridization**: Phosphorus can expand its covalency, but to form Ph5, it would require sp^3d hybridization.
- **Energy Levels**: The energy difference between s, p, and d orbitals is significant. This large energy difference makes the hybridization unfavorable, leading to the conclusion that Ph5 cannot exist.

### Step 3: Analyze BCl6^3-
- **Covalency and Hybridization**: Boron typically has a maximum covalency of 4. To form BCl6^3-, boron would need to expand its covalency to 6.
- **Vacant d-Orbitals**: Boron lacks vacant d-orbitals, which means it cannot accommodate the additional bonding required for the formation of BCl6^3-. Thus, this compound also cannot exist.

### Conclusion
Since all three compounds (NCl5, Ph5, and BCl6^3-) cannot exist due to the reasons discussed, the correct answer is that all of these do not exist.

### Final Answer
**All of these do not exist (Option D).**

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