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 determine in which of the given molecules the vacant orbitals do not participate in bonding, we will analyze each molecule step by step.

### Step 1: Analyze B2H6
- **Structure**: B2H6 (Diborane) has two boron atoms and six hydrogen atoms.
- **Valence Electrons**: Each boron atom has 3 valence electrons (total of 6 from two borons) and each hydrogen has 1 valence electron (total of 6 from six hydrogens).
- **Bonding**: Boron forms 3-center 2-electron bonds (banana bonds) with hydrogen atoms, which involves the use of vacant p-orbitals.
- **Conclusion**: Vacant orbitals participate in bonding.

### Step 2: Analyze Al2Cl6
- **Structure**: Al2Cl6 (Aluminum Chloride) consists of two aluminum atoms and six chlorine atoms.
- **Valence Electrons**: Each aluminum has 3 valence electrons (total of 6 from two aluminums) and each chlorine has 7 valence electrons (total of 42 from six chlorines).
- **Bonding**: Chlorine donates lone pairs to aluminum, which uses its vacant orbitals to accept these electrons.
- **Conclusion**: Vacant orbitals participate in bonding.

### Step 3: Analyze NH3BF3
- **Structure**: NH3BF3 (Ammonia-Boron Trifluoride Complex) consists of ammonia (NH3) and boron trifluoride (BF3).
- **Valence Electrons**: Nitrogen in NH3 has 5 valence electrons (3 shared with H, 2 as lone pairs). Boron in BF3 has 3 valence electrons and forms bonds with three fluorine atoms.
- **Bonding**: The lone pair on nitrogen is donated to the vacant p-orbital of boron.
- **Conclusion**: Vacant orbitals participate in bonding.

### Step 4: Analyze Si2H6
- **Structure**: Si2H6 (Disilane) consists of two silicon atoms and six hydrogen atoms.
- **Valence Electrons**: Each silicon has 4 valence electrons (total of 8 from two silicons) and each hydrogen has 1 valence electron (total of 6 from six hydrogens).
- **Bonding**: Each silicon shares 3 electrons with hydrogen and 1 with the other silicon atom, achieving a stable octet without needing to use vacant orbitals.
- **Conclusion**: Vacant orbitals do not participate in bonding.

### Final Answer
The molecule in which vacant orbitals do not participate in bonding is **Si2H6** (Option 4).

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