Compare $\pi  - $ bond strength between $B$ and $N$ given in two compounds

$(I)$ $\begin{array}{*{20}{c}}
{{{\left( {C{H_3}} \right)}_3}Si - NB{H_2}}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Si{{(C{H_3})}_3}}
\end{array}$     $(II)$ $\begin{array}{*{20}{c}}
{{{\left( {C{H_3}} \right)}_3}C - NB{H_2}}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,|}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,C{{(C{H_3})}_3}}
\end{array}$

Suggest reasons why the $B-F$ bond lengths in $BF_3\, (130\, pm)$ and $BF_4^- \,(143\, pm)$ differ. 

• In the reaction

$2 {X}+\mathrm{B}_2 \mathrm{H}_6 \rightarrow\left\lfloor\mathrm{BH}_2(\mathbf{X})_2\right\rfloor^{+}\left\lfloor\mathrm{BH}_4\right]^{-}$ the amine(s) ${X}$ is$(are)$

$(A)$ $\mathrm{NH}_3$ $(B)$ $\mathrm{CH}_3 \mathrm{NH}_2$ $(C)$ $\left(\mathrm{CH}_3\right)_2 \mathrm{NH}$ $(D)$ $\left(\mathrm{CH}_3\right)_3 \mathrm{~N}$

When $AlCl_3·6H_2O$ is strongly heated, then it forms

Give name of group $13$ elements. 

The incorrect stability order of $+3$ and $+1$ states of $13^{th}$ group elements (boron family) is