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  • Suggest reasons why the B–F bond lengths in BF_3 (130 pm) and BF_4^- (143 pm) differ.

11.14.    Suggest reasons why the B–F bond lengths in BF_{3}  (130 pm)  and  BF_{4}^{-}  (143 pm) differ.

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The B-F bond length in BF_{3} is shorter than the B-F bond length in BF_{4}^{-}. BF_{3} is an electron deficient species, in which p orbital is vacant in Boron. To overcome this problem the fluorine and the boron atom undergoes p\pi-p\pi back bonding. Due to this a partial double bond character is imparts on them. Because of this double bond character, the B-F bond in BF_{3} molecule is shortened.
On the other hand, In the case of BF_{4}^{-}, the hybridization changes from sp^2\rightarrow sp^3. Now boron has 4- sigma bonds and it lose its partial double bond character. Due to this reason, the B-F bond length in BF_{4}^{-} is 143 pm.

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