Question

Suggest reasons why the B–F bond lengths in BF₃ (130 pm) and (143 pm) differ.

Answer 1

The B–F bond length in BF₃ is shorter than the B–F bond length in . BF₃ is an electron-deficient 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) 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 ion.