Suggest reasons why the B–F bond lengths in BF3 (130 pm) and 4 − (143 pm) differ.

Question

Suggest reasons why the B–F bond lengths in BF₃ (130 pm) and BF₄ ⁻ (143 pm) differ. 

Answer 1

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.