Question
Suggest reasons why the B-F bond lengths in $BF_3$ (130pm) and $\text{BF}_4^{-}$ (143pm) differ.
✓
Answer
The B-F bond length in $BF_3$ is shorter than the B-F bond length in $\text{BF}_4^{-}$. $BF_3$ is an electron-deficient species. With a vacant p-orbital on boron, the fluorine and boron atoms undergo $\text{p}\pi-\text{p}\pi$ 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 $BF_3$ (130 pm). However, when $BF_3$ coordinates with the fluoride ion, a change in hybridisation from $sp^2$ (in $BF_3$) to $sp^3$ $\big(\text{in BF}_4^{-}\big)$ occurs. Boron now forms $4\sigma$ bonds and the double-bond character is lost. This accounts for a B-F bond length of 143pm in $\text{BF}_4^{-}$ ion.
This double-bond character causes the bond length to shorten in $BF_3$ (130 pm). However, when $BF_3$ coordinates with the fluoride ion, a change in hybridisation from $sp^2$ (in $BF_3$) to $sp^3$ $\big(\text{in BF}_4^{-}\big)$ occurs. Boron now forms $4\sigma$ bonds and the double-bond character is lost. This accounts for a B-F bond length of 143pm in $\text{BF}_4^{-}$ ion.
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Column II
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