Suggest reasons why the B–F bond lengths in `BF_(3)` (130 pm) and `BF_(4 )^(– )`(143 pm) differ.

Explanation

`BF_(3)` is a planar molecule in which B is `sp^(2)`-hybridized. It has an empty 2p-orbital F-atom has three lone pairs of electron in the 2p-orbitals. Because of similar sizes, `ppi-ppi` back bonding occurs in which a lone pair of electrons is transferred from F to B as shown below :
As a result of this back bonding , `B-F` bond acquires some double bond character . In contrast, in `[BF_(4)]^(-)` ion, B is `sp^(3)`-hybridized and hence does not have an empty p-orbitals available to accept the electrons donated by the F atom. Consequently, in `[BF_(4)]^(-), B-F` is a purely single bond. Since double bonds are shorter than single bonds, therefore, the B-F bond length in `BF_(3)` is shorter (130 pm) than `B-F` bond length (143) pm in `[BF_(4)]^(-)`