The carbon atom of the bromomethane has a partial positive charge and is thus a Lewis acid. The carbon-bromine bond is also not very strong due to the atomic orbital size difference between carbon and bromine. The cyanide ion will be attracted to this positive site (it is a nucleophile…a base). The electron pair from the carbon atom of the cyanide ion will initially place the electron pair in the antibonding orbital (it is empty) of the sigma bond between the carbon atom and the bromine atom.