Polymerization

The tendency of many molecules to aggregate and form dimmers (e.g. Al2Cl6 2), extended polymeric, or larger oligomers structures can be regarded as a donor-acceptor interaction. In the reaction.

a chlorine atom bound to one AlCl3 access nonbonding electrons to complex with the other aluminum atom; as in most other examples of this type the bridging atoms are symmetrically disposed with identical bonds to each aluminum. Polymerization of AXn molecules is more likely to happen when n is small, and when the atom A has vacant orbitals and is big enough to increase its coordination number. Many oxides and halides of stoichiometry AB3 and AB2 form structures that may be regarded as polymeric, although the distinction between an ionic one and this (polar covalent) description is not clear-cut.

Hydrogen bonding will also be regarded as a donor-acceptor interaction in which the acceptor LUMO is the (unoccupied) antibonding orbital of hydrogen bonded to an electronegative element.