Suggest reasons why the BF bond lengths in BF3 (130 pm) and BF4−(143 pm) differ.
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In BF3 molecule, B−F bond has partial double bond due to presence of pπ−pπ backbonding between B and F, which removes the electron deficienty of the molecule. Due to this, B−F bond length is reduced to 130 pm. When BF3 changes to BF−4, the hybridization changes from sp2 to sp3. The double bond character changes to single bond and the B−F bond length becomes 143 pm.