Question

Suggest reasons why the B–F bond lengths in BF3 (130 pm) and BG4– (143 pm) differ.​

Answer 1

The B–F bond length in BF3 is shorter than the B–F bond length in 4-.

BF3is an electron deficient species. With a vacant p-orbital on boron, the fluorine and boron atoms undergo pn–pn 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 hybridization from sp2 (in BF3) to sp3 (in 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 4– ion.

Answer 2

Answer:

In BF3 ‘B’ is sp2 hybridised and in BF4– ‘B’ is sp3 hybridised. Thus, the difference in bond length is due to the state of hybridisation

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