(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

Both `BF_(3)` and `BH_(3)` are electron-deficient compounds. In `BF_(3)`, due to `p pi- p pi` bonding form filled `2p` orbital of `F` to vacant `2p` orbital of `B`, electron deficiency is reduced, thereby increasing the stability of `BF_(3)` molecule.

In `BH_(3)`, due to the absence of lone pair of electron on `H, p pi- p pi` back bonding does not occur , hence, to reduce its electron deficiency, `BH_(3)` dimerises to form `B_(2)H_(6)`.
Therefore, `BF_(3)` exists but `BH_(3)` does not exist.