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Question
Suggest reasons why the B–F bond lengths in BF3 (130 pm) and `"BF"_4^(-)` (143 pm) differ.
Solution
The B–F bond length in BF3 is shorter than the B–F bond length `"BF"_4^(-)`. 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 `"BF"_4^(-)` ) 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 `"BF"_4^(-)` ion.
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