Suggest reasons why the B-F bond lengths in `BF_3` (130 pm) and `BF_4^(-)` (143 pm) differ.

Explanation

The `sp^2` -hybridised B atom in trigonal planar `BF_3` molecule has an empty 2p -orbital. Because of similar sizes of the vacant and filled p -orbitals, `p pi- p pi`back bonding involving transfer of pair of electrons from F to B occurs. As a result, the B-F bond acquires some double bond character. On the other hand, in `BF_4^(-)` ion, the B atom is `sp^3`-hybridised and so, it has no empty p -orbital to accept the electrons donated by the F atom. As a consequence, in `BF_4^(-)` ion, the B-F bond is a purely single bond. Since a double bond is shorter than a single bond, the B-F bond in `BF_3` is shorter in length (130 pm) than the B-F bond (143 pm) in `BF_4^(-)`.