Outer-sphere electron transfer bound

The remarkable physical properties exhibited by the divalent macrobicyclic cage complex [Co(sep)]2+ (29) are unparalleled in Co chemistry.219 The complex, characterized structurally, is inert to ligand substitution in its optically pure form and resists racemization in stark contrast to its [Co(en)3]2+ parent. The encapsulating nature of the sep ligand ensures outer sphere electron transfer in all redox reactions. For example, unlike most divalent Co amines, the aerial oxidation of (29) does not involve a peroxo-bound intermediate. 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

Figure 11.7. Molecular orbital diagram for the oxidation of Fe(II) by O2. An outer-sphere electron-transfer process from v dy to x-jf [or -Kid to irf] is possible. OH bound to Fe(II) can enhance the oxidation by transfer of electron density through both the a and r systems. This stabilizes the Fe(III) formed on oxidation. (From Luther, 1990.)...

Fig. 55. Reaction scheme of superoxide dismutation following the bovine enzyme numbering. The first O2 molecule binds to Cu(II) and is stabilized by the H bond to Arg-141. A second superoxide molecule then approaches the active site and, by an outer-sphere electron transfer via the Cu-bound first O2 molecule, reduces the copper to Cud) 44) (step III). Alternatively, O2" directly reduces superoxide to peroxide 336) (step IV), leaving as dioxygen. Note that the Cu(I)-superoxide and Cu(II)-peroxide complexes are resonant forms of the same molecular arrangement. The newly formed peroxide is protonated by Arg-141 and leaves as HO2. Arg-141 receives a proton from the solvent, restoring the active enz5Tne (I). These reaction proposals do not require the breaking and reforming of the Cu-His-61 bridge.

Similarly, inner-sphere and outer-sphere mechanisms can be postulated for the reductive dissolution of metal oxide surface sites, as shown in Figure 2. Precursor complex formation, electron transfer, and breakdown of the successor complex can still be distinguished. The surface chemical reaction is unique, however, in that participating metal centers are bound within an oxide/hydroxide... 

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