Question

Among following molecule $N-Si$ bond length is shortest:

• A. $N(Si{H}_3{)}_3$
• B. $NH(Si{H}_3{)}_2$
• C. $N{H}_2\left(Si{H}_3\right)$
• D. $\text{All have equal}N-Si\text{bond length}$

Answer 1

The correct option is C $N{H}_2\left(Si{H}_3\right)$

Here in between $N-Si$, back bonding exists which decreases the bond length of $N-Si$ than normal.
As $N-$ atom has one lone pair, more the number of $Si-$atoms attached to $N$, extent of back bonding on each $N-Si$ bond decreases and vice versa.
So, in case of $N{H}_2\left(Si{H}_3\right)$, back bonding is stronger.
More stronger the back bond, smaller will be the bond length.