Metallic pathways, single

Two Pathways. Single electron transfer (SET) from the metal to either the organic halide or the substrate (often, but not always, an aldehyde or a ketone), or perhaps even to both the reactants, is most likely to be the initial step. 

No single mechanism accounts for all the reactions. One pathway involves a concerted one-step process involving a cyclic transition state. This of necessity affords a c -product. Another possibility, more favoured in polar solvents, involves a cationic 5-coordinate intermediate [IrX(A)(CO)L2]+, which undergoes subsequent nucleophilic attack by B-. Other possibilities include a SN2 route, where the metal polarizes AB before generating the nucleophile, and radical routes. Studies are complicated by the fact that the thermodynamically more stable isolated product may not be the same as the kinetic product formed by initial addition. 

The presence of solution at a metal surface, as has been discussed, can significantly influence the pathways and energetics of a variety of catalytic reactions, especially electrocatalytic reactions that have the additional complexity of electrode potential. We describe here how the presence of a solution and an electrochemical potential influence the reaction pathways and the reaction mechanism for methanol dehydrogenation over ideal single-crystal surfaces. 

The water oxidation of CO to C02 via a formate intermediate is well documented, and realistic pathways for reaction (75) at a single metal site are available (437, 438), for example ... 

In the direct transfer mechanism, the metal ion coordinates both reactants enabling an intramolecular reaction, and activates them via polarization. Consequently, strong Lewis acids including Alln and the Lnln ions are the most suitable catalysts in this type of reactions. In the hydride mechanism, a hydride is transferred from a donor molecule to the metal of the catalyst, hence forming a metal hydride. Subsequently, the hydride is transferred from the metal to the acceptor molecule. Metals that have a high affinity for hydrides, such as Ru, Rh and Ir, are therefore the catalysts of choice. The Lewis acidity of these metals is too weak to catalyze a direct hydride transfer and, vice versa, the affinity of Alm and Lnm to hydride-ions is too low to catalyze the indirect hydrogen transfer. Two distinct pathways are possible for the hydride mechanism one in which the catalyst takes up two hydrides from the donor molecule and another in which the catalyst facilitates the transfer of a single hydride. 

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