Oxidation-reduction Marcus mechanism

The last two decades have seen a growing interest in the mechanism of inorganic reactions in solution. Nowhere is this activity more evident than in the topic covered by this review the oxidation-reduction processes of metal complexes. This subject has been reviewed a number of times previously, notably by Taube (1959), Halpern (1961), Sutin (1966), and Sykes (1967). Other articles and books concerned, wholly or partly, with the topic include those by Stranks, Fraser , Strehlow, Reynolds and Lumry , Basolo and Pearson, and Candlin et al ° Important recent articles on the theoretical aspects are those by Marcus and Ruff. Elementary accounts of redox reactions are included in the books by Edwards , Sykes and Benson . The object of the present review is to provide a more detailed survey of the experimental work than has hitherto been available. 

Marcus R. A. (I960), Theory of oxidation-reduction reactions involving electron transfer. Part 4. A statistical-mechanical basis for treating contributions from solvent, ligands and inert salt , Faraday Discuss. Chem. Soc. 29, 21-31. 

Marcus LFER. Oxidation-reduction reactions involving metal ions occur by (wo types of mechanisms inner- and outer-sphere electron transfer. In the former, the oxidant and reductant approach intimately and share a common primary hydration sphere so that the activated complex has a bridging ligand between the two metal ions (M—L—M ). Inner-sphere redox reactions thus involve bond forming and breaking processes like other group transfer and substitution rcaclions, and transition-state theory applies directly to them. In outer-sphere electron transfer, the primary hydration spheres remain intact. The... 

Excited state electron transfer also needs electronic interaction between the two partners and obeys the same rules as electron transfer between ground state molecules (Marcus equation and related quantum mechanical elaborations [ 14]), taking into account that the excited state energy can be used, to a first approximation, as an extra free energy contribution for the occurrence of both oxidation and reduction processes [8]. 

Nonadiabaticity can, in principle, be revealed by deviations from the Marcus cross relation, if the three reactions involved are affected to different extents. In particular, it has been suggested that if reactions of a series of, say, oxidants with a common reductant B are all slightly nonadiabatic, then the self-exchange reaction which is included in the series (the case where A = B) will be favored, since in that case the donor and acceptor orbitals are strictly identical. Free-energy plots suggest that this may indeed be the case for [Co(sep)] and [Fe(H20)6], and for the iron couple, comparisons of self-exchange rates with electrochemical rates have shown a similar anomaly. In all cases the effects are small, and other explanations cannot be ruled out. Indeed, Hupp and Weaver prefer to postulate a special mechanism for the reaction, such as bridging... 

Application of the Marcus cross relation (MCR) to reductions of dioxygen by metal complexes, and to its reverse, oxidations of the superoxide radical anion, 02, supports an outer-sphere mechanism [41 8]. O2 oxidations of organic electron... 

Quantitative rate data for reactions discussed in this section are given in Table 3. Several systems involving the reductant [Ru(NH8)fl] +, which, unless oxidation is slow compared with the loss of NH3, is constrained to react via an outer-sphere mechanism. A test for mechanisms of this type has recently been described in a cross-section correlation which accommodates deviations of a from 0.5 in the Marcus expression, and in the case of the oxidation of... 

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