Complexes, alkyne-metal charge transfer

There has been a growing interest in the utilization of CO2 as a potential Cl source for chemicals and fuels to cope with the predictable oil shortage in the near future. Insertion reactions of CO2 into M-H, M-0, M-N, and M-C bonds are well documented, where these reactions are explained in terms of the electrophilicity of CO2 il, 2). Catalytic syntheses of lactones (3-9) and pyrones (10-16) are also established by incorporation of CO2 into dienes and alkynes activated on low-valent metal complexes. Carbon dioxide shows only an electrophilicity under usual reaction conditions, but it exhibits a nucleophilicity upon coordination to low-valent metals because of the intramolecular charge transfer from metals to CO2. Metal-C02 formation may be the key species in electro- and photochemical CO2 reductions. Since the first characterization of [Ni(PCy3)2(T) (C,0)-C02)] (17), a variety of metal... 

Unactivated, unsaturated hydrocarbons are not routinely subject to attack by nucleophiles. Complexation of alkenes, alkynes, or arenes to a positively charged metal center may lead to activation of the hydrocarbyl group towards nucleophilic attack and, in particular, to addition of hydride ion. The effect of the [LnM] fragment on the reactivity of the unsaturated ligand is largely due to the transfer of electron density from the unsaturated hydrocarbyl to the metal center upon coordination, a process that is most effective when the metal center is positively charged. Figure 10.5 ... 

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