(a) Explain why `BF_(3)` exists whereas `BH_(3)` does not.
(b) Compare the B-F bond length in `BF_(3)` and `BF_(4)^(-)`

Explanation

Due to `p pi- p pi ` back bonding, the lone pair of electrons of F is denoted to the B-atom. This delocalisation 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. `BH_(3)` dimerises to form `B_(2)H_(6)`
(b) Because of double bond character of B-F bonds in `BF_(3)`, it has shorter B-F bond length than in `BF_(4)^(-)`