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BF3 behaves as a Lewis acid because centralboron atom has only six electrons (three pairs) after sharing with theelectrons of the F atoms. It is an electron deficient compound and, therefore,behaves as a Lewis acid.
Boron Halides. The halides of boron are covalent in naturebecause the central boron atom, as stated earlier, is very small in size andcannot part with the valence electrons to give a trivalent B3+ Thus, BF3, BCl3, BBr3 and BI3 are all covalent in nature.
In these halides, the central B atom is sp2 hybridised with a vacant 2p orbital (1 .v2 2s1 2pxl2pyi2pz°). The hybridised orbitals are directedtowards the three corners of an equilateral triangle and are involved incovalent bond formation with the half filled p-orbitals of the halogen atoms (ns2p5).
The empty 2p orbital lies perpendicular to thehybridised orbitals. Since it is empty, it can accept an electron pair fromelectron donor species (Lewis bases) resulting in co-ordinate or dative bond formation. Withthree shared pairs of electrons on the central boron atom, boron halidesare Lewis acids i.e., electron deficient molecules and take part in Lewis acid– base reactions. In the compounds thus formed, boron atom undergoes a changein state of hybridisation from sp2 to sp3. These are tetrahedral in nature.
