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Carbon-Hydrogen bond activation
Chemistry Central Journalvolume 3, Article number: P46 (2009)
In early experimental studies of transition-metal activation of methane two alternative mechanisms were invoked: an exchange reaction via a sigma-bond metathesis route and the oxidative addition of methane to form a stable metal methyl hydride. Computational chemistry has provided considerable insight into details of the mechanisms operating in these bond activation reactions. For the low-temperature activation of alkanes by [Cp*Ir(PMe3)(CH3)]+ density functional calculations predict an oxidative-addition pathway with a stable Ir(V) intermediate. Both of these mechanisms appear to be accessible for C-H activation by the reactive carbene intermediate Cp*W(= CRH)L, L = NO, CO. Recently, in cases where the optimized transition states appear to resemble geometrically an oxidative-addition intermediate, various research groups have suggested alternative C-H activation mechanisms. A Bader analysis of these transition states is capable of resolving these alternatives and classifying them into a range of mechanisms. Recently, time resolved infrared spectroscopy and density functional calculations have revealed new details about the intermediates and transition states involved in the oxidative-addition reaction of alkanes by photochemically generated TpRh(CO) and CpRh(CO).