Electron transfer via dithiaspiroalkane linkages. Nature of long-range through-bond electronic coupling in disulfoxide radical cations and bis(metal) complexes and implications for the characterization of the sulfoxide bond

Electron transfer in bis(ruthenium pentaammine) complexes III—V of the single-, double-, and triple-ring dithiaspiroalkanes is known to be surprisingly facile. The mechanism of donor-acceptor interaction with these rigid nonconjugated bridges is thus of considerable interest. The electronic structure of the series of disulfoxides VI and VII for which photoelectron spectroscopic data are available has been studied in order to investigate this. Ab initio SCF calculations using a variety of basis sets have been carried out, and the ionization potentials (IP's) at Koopmans' theorem, ΔSCF, and projected broken-symmetry levels of approximation, obtained. Calculations were also performed for the related dithiaspiroalkanes, for the single-ring sulfide X and monosulfide IX, for dimethyl sulfoxide, and for ethylene sulfoxide, at the experimental or interpolated geometries. The use of a polarized (3-21G*) rather than an unpolarized (3-21G) basis for IX and the double-ring disulfoxide VI resulted in comparatively small changes in calculated IP's and orbital character. Thus, orbital assignments obtained without the inclusion of 3d functions are judged to be justified. The SO bond is then considered as an essentially semipolar link (>S^δ+—O^δ-) rather thanas one involving hypervalent S (>S═O). The observed near constancy of the first IP of dithiaspiroalkanes and their mono- and disulfoxide derivatives is shown to be accidental. The trends in the experimental IP's are well reproduced by calculations, and the nature of the through-bond interaction is related to orbital type. An SO bond lying out of the plane of the thiaspiroalkane ring is necessary for nonzero interaction. Implications for electron transfer in metal complexes and relationships with transfer through rigid hydrocarbon chains are discussed. Through-space effects are equally important in the radical cation of the double-ring molecule VI.