The B–F bond length in BF3 is shorter than the B–F bond length in BF4 − . BF3 is an electrondeficient species. With a vacant p-orbital on boron, the fluorine and boron atoms undergo pπ–pπ 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 BF3 (130 pm). However, when BF3 coordinates with the fluoride ion, a change in hybridisation from sp2 (in BF3) to sp3 (in BF4 −) occurs. Boron now forms 4σ bonds and the double-bond character is lost. This accounts for a B–F bond length of 143 pm in BF4 − ion.