Both `N(SiH_(3))_(3) and NH(SiH_(3))_(2)` compounds have trigonal planar skeleton. Incorrect statement about both compounds is :

Options

A. `SiNSi` bond angle in `NH(SiH_(3))_(2)gtSiNSi` bond angle in `N(SiH_(3))_(3))`
B. `N-Si` bond length in `NH(SiH_(3))_(2)gtN-Si` bond length in `N(SiH_(3))_(3))`
C. `N-Si` bond length in `NH(SiH_(3))_(2)ltN-Si` bond length in `N(SiH_(3))_(3))`
D. Back bonding strength in `NH(SiH_(3))_(2)gt` Back bonding strength in `N(SiH_(3))_(3))`

Explanation

To solve the problem regarding the compounds \( N(SiH_3)_3 \) and \( NH(SiH_3)_2 \) and identify the incorrect statement about them, we will analyze the structures and bonding characteristics of both compounds step by step.

### Step 1: Understand the Structures
1. **Compound 1: \( N(SiH_3)_3 \)**
- This compound has a nitrogen atom bonded to three \( SiH_3 \) groups.
- The nitrogen has a lone pair of electrons, which can participate in back bonding with the silicon atoms.
- The geometry around the nitrogen is trigonal planar due to the three \( SiH_3 \) groups and the lone pair.

2. **Compound 2: \( NH(SiH_3)_2 \)**
- This compound has a nitrogen atom bonded to one hydrogen atom and two \( SiH_3 \) groups.
- Similar to the first compound, the nitrogen can also engage in back bonding with the silicon atoms.
- The geometry around the nitrogen is also trigonal planar.

### Step 2: Analyze Bonding and Angles
- **Back Bonding:** In both compounds, back bonding occurs between the lone pair on nitrogen and the vacant p-orbitals of silicon. This back bonding stabilizes the structure and influences bond angles.
- **Bond Angles:** The bond angles in both compounds are expected to be approximately 120 degrees due to the trigonal planar arrangement.

### Step 3: Evaluate the Statements
We need to evaluate the statements provided in the question.

1. **Statement A:** The Si-Si bond angle in \( NH(SiH_3)_2 \) is greater than the Si-Si bond angle in \( N(SiH_3)_3 \).
- This statement is **correct** because the back bonding in \( NH(SiH_3)_2 \) is distributed among two \( SiH_3 \) groups, leading to stronger bond angles.

2. **Statement B:** The N-Si bond length in \( NH(SiH_3)_2 \) is greater than the N-Si bond length in \( N(SiH_3)_3 \).
- This statement is **incorrect**. The bond strength due to back bonding is greater in \( N(SiH_3)_3 \) because the lone pair is shared among three \( SiH_3 \) groups, resulting in a shorter N-Si bond length.

3. **Statement C:** The N-Si bond length in \( N(SiH_3)_3 \) is less than the N-Si bond length in \( NH(SiH_3)_2 \).
- This statement is **correct** and aligns with the explanation above.

4. **Statement D:** The back bonding strength in \( NH(SiH_3)_2 \) is greater than in \( N(SiH_3)_3 \).
- This statement is **correct** as the back bonding is stronger in \( N(SiH_3)_3 \) due to the involvement of three \( SiH_3 \) groups.

### Conclusion
The incorrect statement about both compounds is **Statement B**.