The empty p orbital on the boron makes it susceptible to nucleophilic attack. Alkyl migration can then occur if the nucleophile has a leaving group or electron sink of some sort – this process can be very useful in syntheses.
This pattern of addition-then-migration dominates the reactions of alkylboranes.
Full cis addition of H2 to an alkene – an alternative to the metal catalysed methods described in the electrophilic addition chapter.
This can also be carried out on alkenylboranes (which are formed with alkynes plus B-H).
This also works for I and Br.