Alkenes that contain electron-withdrawing substituents also act as acceptor
molecules, as do carbon tetrahalides95 and certain anhydrides.96 A particularly
strong alkene acceptor is tetracyanoethylene.97
The bonding in these cases is more difficult to explain than in the previous
case, and indeed no really satisfactory explanation is available.98 The
difficulty is that although the donor has a pair of electrons to contribute
(both n and p donors are found here), the acceptor does not have a vacant
orbital. Simple attraction of the dipole-induced dipole type accounts for some
of the bonding,99 but is too weak to explain the bonding in all cases;100 for
example, nitromethane, with about the same dipole moment as nitrobenzene,
forms much weaker complexes. Some other type of bonding clearly must also
be present in many EDA complexes. The exact nature of this bonding, called
charge-transfer bonding, is not well understood, but it presumably involves
some kind of donor–acceptor interaction.
Crown