Class 11 Chemistry - Chapter The p Block Elements NCERT Solutions | Suggest reasons why the B-F bond lengths
Suggest reasons why the B-F bond lengths in BF3 (130 pm) and BF-4(143 pm) differ.
The B-F bond length in BF3 is shorter than the B-F bond length in BF-4 . BF3 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 4 σ 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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