`BF_(3)` exists as discrete molecules but `BH_(3)` exists as dimer. Explain .

Explanation

Due to `ppi-ppi` backbonding, the lone pair of electrons of F is donated to the B-atom. This delocalization reduces the deficiency of electrons on B thereby increasing the stability of `BF_(3)` molecule.

Due to absence of lone pair of electrons on H-atom, this compensation does not occur in `BH_(3)`. In other words, electron deficiency of B stays and hence to reduce its electron deficiency, `BBH_(3)` dimerises to form `B_(2)H_(6)`.