Inner-sphere electron transfer in metal-cation chemistry

Bond activation of organic molecules by metal cations is usually rationalized either in terms of the chemistry of Lewis acids or via oxidative addition of metal fragments to R-X bonds, that is, R-X+M⁺→R-M⁺-X→ products. In most of these mechanisms, the positive charge is assumed to be located on the metal center. Here, we propose an alternative mechanism, to which we refer as inner-sphere electron transfer (ET). Thus, provided that certain conditions are fulfilled, the insertion species R-M⁺-X may isomerize via ET to [R⁺···MX] structures with the positive charge located mostly at the organic residue R. If R-M⁺-X and [R⁺···MX] are not just resonance structures but distinct minima on the related potential-energy surfaces, there also exists a transition structure between the two, that is, an ET-TS. Here, the role of inner sphere ET in organometallic gas-phase reactions and the possible existence of ET-TSs are discussed for a series of examples investigated both computationally and experimentally.