The B–F bond length in `BF_(3)` is shorter than the B–F bond length in . `BF_(3)BF_(4)^(-)` is an electron-deficient species. With a vacant p-orbital on boron, the fluorine and boron atoms undergo `ppi–ppi` back-bonding to remove this deficiency. This imparts a double- bond character to the B–F bond.
This double-bond character causes the bond length to shorten in `BF_(3)` (130 pm). However, when `BF_(3)` coordinates with the fluoride ion, a change in hybridisation from `sp^(2)`
`("in" BF_(3))` to `sp^(3)` `("in" BF_(4)^(-))` occurs. Boron now forms `4sigma` bonds and the double-bond character is lost. This accounts for a B–F bond length of 143 pm in `overset(BF_(4)^(-))(ion)`.
