Which of the following is known as inorganic benzene?

$A$ compound $X$,of boron reacts with $NH_3$ on heating to give another compound $Y$,which is called inorganic benzene. The compound $X$ can be prepared by treating $BF_3$ with lithium aluminium hydride. The compounds $X$ and $Y$ are represented by the formulas.

Which of the following does not liberate $H_2$ gas upon reaction with an acid?

Boron trifluoride on reaction with lithium aluminium hydride in ether gives $LiF$, $AlF_3$ and $X$. $X$ on reaction with $NH_3$ gives $Y$. $Y$ on further heating gives a compound $Z$. The number of $\sigma$-bonds and $\pi$-bonds in $Z$ are $x$ and $y$ respectively. $(x+y)$ is equal to

The composition of borax is:

Which elements of group $13$,except $B$,can form tetrahedral and octahedral complexes in aqueous medium?