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4305where the FeIlTPP is formed so that no catalytic reduction ensues until the potential approaches 0.1 V where the Fe"TPP-0, adduct is reducible. (The final wave in Figure 12C arises from the reduction of the second form of adsorbed FeTPP that does not participate in the catalysis.)The behavior shown in Figures 9F, 10, and 12 demonstrates the lack of reduction of O2 by the FeIITPP which is generated at potentials positive of 0.1V. This behavior provides strong evidence against an alternative reaction mechanism in which the first step following the electroreduction of the catalyst is simple, outer-sphere electron transfer between Fe"TPP and 02. Such a mechanism has been proposed for the catalysis of O2 reduction by iron porphyrins in solution? but it can be ruled out for adsorbed FeTPP on the basis of the reaction pattern shown in Figures 9F, IO, and 12. The behavior of the adsorbed FeTPP and FeTMPyP catalysts, in which reduction of the adsorbed catalyst occurs at potentials ahead of those where the catalyzed reduction of O2 proceeds, is the same as that exhibited by the corresponding cobalt porphyrin, CoTPP, where the reduction of ColIITPP to Co"TPP occurs at potentials much more positive than those where CoTPP serves as a catalyst for the reduction of 02." Thus, the behavior (33) Ni, C.-L.; Anson, F. C. Inorg. Chem. 1985, 24, 4754.of iron and cobalt porphyrins in catalyzing the electroreduction of O2 can be rationalized by means of a single reactivity pattern:The catalytic reduction of O2 can occur at potentials no more positive than that at which the metal center in the metalloporphyrin is reduced from the +3 oxidation state, which does not interact with 02, to the +2 oxidation state, which does. For cobalt porphyrins, this potential is much more positive than that where the electroreduction of O2 is observed. The latter potential corresponds to the intrinsic reduction potential of the cobalt porphyrin4ioxygen adduct, which is evidently significantly more negative than the formal potential of the C O~I~/ C O~~ couple of the cobalt porphyrin. In the case of iron porphyrins, E2 (reaction 4) is more positive than E , (reaction 2) (except at low pH values). The result is a close correspondence between the potential where the iron(II1) center in the catalytically relevant adsorbed porphyrin is reduced in the absence of O2 and that where the catalyzed reduction of O2 appears. The apparent mismatch between these two potentials that was emphasized in previous studies'O of several iron porphyrins has been shown in the present work to disappear when adequate account is taken of the effect of adsorption on graphite on the formal potentials of the iron porphyrins. Two synthetic methods are described to produce platinum(0) complexes that contain two metal atoms held together by bridging phosphine ligands. Both platinum(1) and platinum(l1) complexes are reduced by using chemical and electrochemical techniques to produce the desired platinum dimers. The electrochemistry and the solution chemistry of these complexes are inve...