X-ray structures and electronic spectra of the π-halogen complexes between halogen donors and acceptors with π-receptors

The spectral characteristics and thermodynamics for the formation of intermolecular complexes of dihalogens, halocarbons, and halide anions with various organic (aromatic and olefinic) pi-receptors show their direct relationship to the more traditional donor/acceptor complexes. The unified Mulliken dependence indicates the common (charge-transfer) origin of the long-distance pi-bonding of the halogen centers. X-ray structural analyses of the molecular complexes of various arene donors with dihalogen (X-2) and halocarbon (XR) acceptors reveal that such non-covalent interactions are characterized mostly by over-the-rim halogen coordination, with the intermolecular halogen-carbon contacts shortened by about 0.3-0.4 angstrom relative to the sum of the van der Waals radii. The location of dihalogen and halocarbon acceptors generally follow the position of highest electron density on the aromatic ring, and the X-X or X-R bonds are directed perpendicular to the aromatic planes. Likewise, the structural arrangements of the halide donors in the molecular complexes with cyano-substituted arenes (by and large) follow the LUMO shape of the aromatic acceptor. Such long-distance pi-bondings are attributed to the presence of multiple (close-lying) local energy minima, so that the donor/acceptor arrangements are rather easily modulated by external forces.