Observations on the Crystal Structures of Electron Donor‐Acceptor Complexes

The theoretical interpretation of electron donor‐acceptor complex formation in terms of charge transfer interactions has stimulated many structure determinations for these complexes. These fall into three classes, depending on the type of orbitals involved in charge transfer. In σ‐σ complexes, intermolecular bonds become shorter and intramolecular bonds become longer as charge transfer increases. Relative orientations correspond to overlap of donor and acceptor molecules in directions of “preferred polarizability”. Intermolecular bond lengths in σ‐π complexes show similar trends, and the axial orientation in the benzene‐halogen complexes is probably the result of the best compromise between orbital overlap and energy factors. π‐π Complexes contain stacks of alternate plane‐to‐plane donor and acceptor molecules, arranged in three characteristic ways. There is little correlation between interplanar spacing in these stacks and charge transfer properties. The relative orientations of donor and acceptor molecules within the stacks are determined by a combination of charge transfer interactions (maximized when aromatic rings of donor and acceptor molecules are displaced by half a ring diameter) and dipole‐induced dipole interactions (maximized, for example, when a polar bond of one molecule overlaps a polarizable region of another). Crystal packing requirements and dispersion forces modify these effects, and no satisfactory theoretical treatment of this complex combination of interactions is yet available.