In which of the following compounds observed bond angle is found to be greater than expected, but not due to back bonding.

Options

A. `N(SiH_(3))_(3)`
B. `N(CH_(3))_(3)`
C. `O(CH_(3))_(2)`
D. `O(SiH_(3))_(2)`

Explanation

To solve the question, we need to analyze the bond angles in the given compounds and determine which ones exhibit an observed bond angle greater than expected, but not due to back bonding.

### Step-by-Step Solution:

1. **Understanding the Question**: We need to find compounds where the observed bond angle is greater than the expected bond angle, but this increase is not due to back bonding.

2. **Evaluating Each Option**:
- **Option 1: N(SiH3)3**
- Nitrogen has 5 valence electrons and forms three bonds with SiH3 groups. The expected bond angle is influenced by a lone pair on nitrogen.
- However, the lone pair participates in back bonding with the vacant orbitals of silicon, leading to an increase in bond angle due to this interaction.
- **Conclusion**: This option cannot be the answer because the increase is due to back bonding.

- **Option 2: N(CH3)3**
- Here, nitrogen is bonded to three methyl groups. The expected bond angle in a tetrahedral geometry is around 109.5 degrees.
- Due to steric repulsion between the bulky methyl groups, the observed bond angle increases to approximately 110.9 degrees.
- **Conclusion**: This increase is not due to back bonding but due to steric repulsion, making this a valid option.

- **Option 3: O(CH3)2**
- In this case, oxygen is bonded to two methyl groups. The expected bond angle in a tetrahedral arrangement is around 109.5 degrees.
- The observed bond angle is around 110 degrees, which is greater than expected due to steric repulsion between the methyl groups.
- **Conclusion**: Similar to option 2, this increase is not due to back bonding, making this another valid option.

- **Option 4: O(SiH3)2**
- In this compound, oxygen is bonded to two SiH3 groups. The expected bond angle is influenced by the presence of vacant d-orbitals on silicon, which can engage in back bonding.
- The bond angle increases due to this back bonding effect.
- **Conclusion**: This option cannot be the answer because the increase is due to back bonding.

3. **Final Answer**: The compounds where the observed bond angle is greater than expected but not due to back bonding are **Option 2 (N(CH3)3)** and **Option 3 (O(CH3)2)**.