Single versus Multi-Electron Processes

It is clear that the electron stoichiometry strongly affects the reaction mechanism and therefore the potentials required. As demonstrated by the natural photosystems, properly designed co-catalysts can circumvent these high-energy one-electron path- 

MacDonnell and coworkers have shown that compounds 11 and 23 undergo two and four electron reductions to yield 22 and 24, respectively, under photochemical conditions in both MeCN and water (see Fig. n),208-210 (n this system, the photoreductions are ligands based and are seen to occur as stepwise one-electron processes under basic conditions. At lower pH s (-6-8), protonation of the reduced cen- 

As shown previously in Fig. 9, compound 16 is a promising addition to this family of complexes capable of photodriven multi electron processes. Flash photolysis reveals a stepwise three electron oxidation at the MnnMn center to yield the MnlnMnIV complex 17.196,197 Unlike the preceding examples, multiple oxidizing equi valents or holes are stored during the photochemical reaction making this system complimentary to those that collect multiple electrons. 

Bocarsly, Pfennig and co workers reported interesting multi electron photoreac tions for the trimeric M Ptlv M complexes 25a-c.212 215 In this system, a single photon excitation into the intervalence charge transfer band results dissociation of the trimer into [Pt(NH3)4]2+ and two equivalents of a M111. The initial photoexcited complex is though to dissociate first to a Mm complex and Ptni-Mn intermediate. The latter dimer subsequently undergoes a thermal electron transfer reaction to yield the final products. 

In a related fashion, Haga and coworkers showed that the tetranuclear [Rm]8+ complex 26 is doubly reduced by a combination of photo- and thermal induced re 

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Several studies have been conducted aiming ai the separation of steric and electronic effects [71-73]. For a single step process such as an oxidative addition or one-electron change in electrochemical processes this may be useful, but for multi-step reactions as we are dealing with in catalysis, this technique will encounter many problems. There will be different effects on the distinct steps and linear free-energy relationships will be an exception rather than the rule. When for instance both an oxidative addition step and a reductive elimination step are involved volcano curves must be expected for reactivity versus a ligand property, as in a series of metal oxides when... 

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