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Suggest reasons why the B - F bond lengths in BF_3 (130 pm) and BF_4^- (143 pm) differ. |
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Answer» Solution :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. With a vacant p-orbital on boron, the FLUORINE and boron atoms undergo `ppi-ppi` 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` (in `BF_4^-`) 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 `BF_4^-` ion. 
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