Bridging, intermolecular electron-deficient

Boron fonns the trifluoride molecule, BF, which is planar. If the boron atom simply exerted its trivalence, the B-F bonds would be straightforward single covalent bonds. The bonds are shorter than expected for single bonds, and the boron atom would not have a share in an octet of electrons. As in BeF the deficiency of electrons is made up by the use of a lone pair of electrons from a fluorine atom, but the process is intramolecular l alher than the intermolecular bridging in BeF. One fluorine atom donates an electron pair to the central boron atom to make up the octet. All three B-F bonds are identical, which is explained in terms of valence bond theory by the canonical forms shown in Figure 7.2. Molecular orbital theory retains the three single bonds, but has an extra pair of fluorine 2p electrons delocalized over the molecule evenly to give the same effect. 

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