Kinetic analysis of the successive oxidative cyclic voltammetric responses of [Os II (bpy)2py(OH2)] 2؉ in buffered water, together with determination of H/D isotope effects, has allowed the determination of the mechanisms of the successive proton-coupled electron transfers that convert the Os II -aquo complex into the Os III -hydroxo complex and the later into the Os IV -oxo complex. The stepwise pathways prevail over the concerted pathway in the first case. However, very large concentrations of a base, such as acetate, trigger the beginning of a concerted reaction. The same trend appears, but to a much larger extent, when high local concentration of carboxylates are attached close to the Os complex. The Os III -hydroxo/Os IV -oxo couple is globally much slower and concerted pathways predominate over the stepwise pathways. Water is, however, not an appropriate proton acceptor in this respect. Other bases, such as citrate or phosphate, are instead quite effective for triggering concerted pathways. Here, we suggest factors causing these contrasting behaviors, providing a practical illustration of the prediction that concerted processes are an efficient way of avoiding high-energy intermediates. Observation of a strong decelerating effect of inactive ions together with the positive role of high local concentrations of carboxylates to initiate a concerted route underscores the variety of structural and medium factors that may operate to modulate and control the occurrence of concerted pathways. These demonstrations and analyses of the occurrence of concerted pathways in an aquohydroxo-oxo series are expected to serve as guidelines for studies in term of methodology and factor analysis.concerted processes ͉ electron transfer chemistry of metal complexes M any reactions of electron transfer chemistry and radical chemistry associate single electron transfer with proton transfer. Molecular electrochemists have been interested in this as a special case of the coupling between electron transfer and acid-base reactions in a general sense (ref. 1, pp. 140 -148). In these electrochemical proton-coupled electron transfers (PCET), proton transfer and electrochemical electron transfer have been viewed for a long time as separate steps as represented by the blue pathways in Scheme 1, proton transfer first, followed by electron transfer (PET) or vice versa (EPT). The mechanisms of many important processes have thus been uncovered by observing the change induced by addition of an acid to a nonacidic solvent in reductions or, conversely, of a base to a nonbasic solvent in oxidations (1). In water, treatment of the kinetics under the assumption that the protonation step are fast and reversible, albeit not always relevant, has proved successful in quite a number of cases (2). This conceptual framework changed rapidly when it was realized that electrochemistry could contribute efficiently to the interpretation of clues indicating that proton and electron transfers could be concerted (red pathway in Scheme 1) in several natural process...

The reductive cleavage of chloro- and polychloroacetamides in N,N-dimethylformamide gives new insights into the nature of the in-cage ion radical cluster formed upon dissociative electron transfer. Within the family of compounds investigated, the electrochemical reduction leads to the successive expulsion of chloride ions. At each stage the electron transfer is concerted with the breaking of the C-Cl bond and acts as the rate-determining step. The reduction further leads to the formation of the corresponding carbanion with the injection of a second electron, which is in turn protonated by a weak acid added to the solution. From the joint use of cyclic voltammetric data, the sticky dissociative electron-transfer model and quantum ab initio calculations, the interaction energies within the cluster fragments (*R, Cl-) resulting from the first electron transfer to the parent RCl molecule are obtained. It is shown that the stability of these adducts, which should be viewed as an essentially electrostatic radical-ion pair, is mainly controlled by the intensity of the dipole moment of the remaining radical part and may eventually be strengthened by the formation of an intramolecular hydrogen bond, as is the case with 2-chloroacetamide.