`R-overset(+)(N)-=C:^(-)`. Although N has a +ve charge but in cannot accept a pair of electrons because it has already 8 electrons in the valence shell and cannot expand its valence shell beyond 8 due to the absence of d-orbitals. Therefore, it cannot act as an electrophile. However, due to the presence of a -ve charge on carbon, it is easily donate a pair of electrons and hence can act as a nucleophile.
