Boron fluoride exists as $BF_3$ but boron hydride does not exist as $BH_3$. Give reason. In which form does it exist? Explain its structure.

(N/A) In $BF_3$,the lone pair of electrons on the fluorine atom is donated to the vacant $p$-orbital of the boron atom through $p\pi-p\pi$ back bonding. This delocalization of electrons reduces the electron deficiency of the boron atom,thereby decreasing its Lewis acidic character and increasing the stability of $BF_3$.
In $BH_3$,there is no lone pair of electrons on the $H$ atom to participate in back bonding. Therefore,$BH_3$ dimerizes to form $B_2H_6$ (diborane) to achieve stability.
Structure of $B_2H_6$: In diborane,the four terminal $H$ atoms and two $B$ atoms lie in the same plane. The two bridging $H$ atoms lie above and below this plane. Each boron atom is $sp^3$ hybridized. The bridging $H$ atoms are involved in $3$-center-$2$-electron $(3c-2e)$ bonds,also known as banana bonds.