Ncert Exercises & Solutions

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

The B–F bond length in $B F_{3}$ is shorter than the B–F bond length in $B {F_{4}}^{-}$ . $B F_{3}$ 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 $B F_{3}$ (130 pm). However, when $B F_{3}$ coordinates with the fluoride ion, a change in hybridisation from $s p^{2}$ (in $B F_{3}$ ) to

$s p^{3}$
(in $B {F_{4}}^{-}$ ) 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 $B {F_{4}}^{-}$ ion.

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