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Suggest reasons why the B-Fbond lengths in BF_(3) (130 pm) and BF_(4)^(-) (143 pm) differ. |
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Answer» Solution :`BF_(3)` is a planar MOLECULE in which B is `sp^(2)`-hybridized. It has an empty 2p-orbital F-atom has threelone pairs of electronin the 2p-orbitals. Because of similar sizes, `ppi-ppi` back bondingoccurs in whicha lone pair of electrons is transferredfrom F to B as shownbelow : As a result of this back bonding , `B-F`bond acquiressome doublebond character . In contrast, in `[BF_(4)]^(-)`ION, B is `sp^(3)`-hybridizedand hencedoes not havean empty p-orbitalsavailableto acceptthe electronsdonatedby the F atom. Consequently, in `[BF_(4)]^(-), B-F` is a purely SINGLE bond. Sincedoublebonds areshorter than singlebonds,therefore, the B-F bond length in `BF_(3)` is SHORTER (130 pm) than `B-F` bond length(143) pm in `[BF_(4)]^(-)` 
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