In which of the folowing molecules, vacant orbitals do not participate in bonding :-

Options

A. `B_(2)H_(6)`
B. `Al_(2)Cl_(6)`
C. `[H_(3)N.BF_(3)]`
D. `Si_(2)H_(6)`

Answer

To solve the question regarding which of the following molecules has vacant orbitals that do not participate in bonding, we will analyze each molecule based on the size of the atoms involved.

### Step-by-Step Solution:

1. **Understanding the Concept**:
- Vacant orbitals participate in bonding when the sizes of the atoms involved are comparable. If one atom is significantly larger than the other, the vacant orbitals of the larger atom may not effectively overlap with the orbitals of the smaller atom.

2. **Analyzing the First Molecule (V2H6)**:
- V2H6 consists of vanadium (V) and hydrogen (H).
- The size of vanadium and hydrogen is comparable.
- Therefore, the vacant orbitals of vanadium will participate in bonding.
- **Conclusion**: This cannot be our answer.

3. **Analyzing the Second Molecule (Al2Cl6)**:
- Al2Cl6 consists of aluminum (Al) and chlorine (Cl).
- The sizes of aluminum and chlorine are also comparable.
- Thus, the vacant orbitals of aluminum will participate in bonding.
- **Conclusion**: This cannot be our answer.

4. **Analyzing the Third Molecule (NH3.Bx3)**:
- NH3.Bx3 consists of nitrogen (N), boron (B), and hydrogen (H).
- The sizes of nitrogen, boron, and hydrogen are comparable.
- Hence, the vacant orbitals of boron will participate in bonding.
- **Conclusion**: This cannot be our answer.

5. **Analyzing the Fourth Molecule (Si2H6)**:
- Si2H6 consists of silicon (Si) and hydrogen (H).
- The size of silicon is much larger than that of hydrogen.
- Due to this significant size difference, the vacant orbitals of silicon will not effectively overlap with the hydrogen orbitals.
- **Conclusion**: The vacant orbitals of silicon do not participate in bonding.

6. **Final Answer**:
- Based on the analysis, the correct answer is **Option D: Si2H6**.