Welcome to ChemZipper !!!: Why B-F bond length in BF3 is shorter (130 pm) than B-F bond Iength in BF4- (143 pm)?. Explain.

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Why B-F bond length in BF3 is shorter (130 pm) than B-F bond Iength in BF4- (143 pm)?. Explain.

BF3 has a planar trigonal geometry due to sp2 hybridisation of boron in BF3. Boron atom in BF3 is electron defficient (incomplete octet) and fluorine atom is electron rich (due to the presence of three lone pair of electrons on fluorine atom). Boron and fluorine both belong to the same period. Hence their bonding orbitals have almost same size  and same energies.Thus back bonding takes place between B and F. This introduces a partial double bond  character between B-F of BF3. Thus B-F bond length decreases from normal B-F bond length.

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