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Boron has only three valence electrons so typically forms only three conventional two-centre two-electron bonds with other atoms in a planar structure leaving a vancant 2p orbital. The vacant orbital is able to accept a lone pair of electrons from a Lewis base to give a neutral species or can combine with a nucleophile to form a negatively charged tetrahedral anion.
One of the simplest classes of nucleophiles that attacks borane is that of alkenes. The result, described as hydroboration is an overall addition of borane across the double bond. CLICK the 'hydroboration' and 'oxidation' links below to view both of these reactions respectively: