σ* Radicals formed by electron-capture and electron-loss

Abstract

σ* Radicals (A•-B-), sometimes called three-electron-bonded radicals, are now known to be important intermediates in a range of reactions. In this review, general methods for preparation and study are described, and it is shown how EPR spectroscopy can be used to obtain very detailed information about the SOMO for these species. This is shown to be strongly confined to the two atoms that are directly involved in the σ* orbitals. Examples are drawn from homonuclear species (A•-A-), which are generally the most stable, and from heteronuclear species (A•-B-). (The negative charge used herein is arbitrarily selected for illustrative purposes only: these radicals may, in fact, be negative, neutral or positive). For the former, the 'parent' σ*-radical, H2- leads on to the VK centres (e.g., Cl2-), RS•-SR- and R2S'-SR2+, R3P-PR3+ and certain SiSi- centres. For the latter, discussion moves from the limiting case of R3C'-hal-, (where R=alkyl), which is better represented as R3C•hal- with a very weak, 'charge transfer' interaction, to the isoelectronic R3N•-hal species which are clearly σ* radicals. Qualitative reasons for this difference are outlined. Other species discussed are hydrogen derivatives, H÷X, the species RS•-SR2, which may possibly be important in organosulfur chemistry and a novel species formed from the salt Ph3PH+Br-. © Acta Chemica Scandinavica 1997.