Explain why `BF_3` exists whereas `BH_3` does not.

Explanation

Because of `p pi- p pi` backhanding, the lone pair of electrons of F is donated to the B -atom. This delocalisation reduces the deficiency of electrons on B -atom and as a consequence, the stability of `BF_3` molecule increases.

Due to absence of lone pair of electrons on H -atom, similar compensation does not take place in `BH_3` . In other words, deficiency of elecron on B retains and therefore, to reduce its electron deficiency, `BH_3` dimerises to form `B_2H_6` (diborane).