Structures
When boron forms three covalent bonds, as in the borate ion, BO33-, it has a share in three pairs of electrons. VSEPR theory predicts a trigonal planar structure as this is the configuration that minimizes the energy of repulsion between electrons. It also predicts that in compounds where the boron atom bonds with three oxygen atoms the inter-bond angle will be 120°. In terms of valence bond theory the bonds are formed by using sp2 hybrid orbitals. The fact that there is an incomplete octet means that these compounds are Lewis acids.
When a trigonal boron atom accepts a pair of electrons from a Lewis base, it adopts a tetrahedral configuration (sp3), and the octet rule is satisfied. Both trigonal and tetrahedral units can co-exist in a complex borate, such as the anion in borax (crystal structure below).
Read more about this topic: Borate
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