Electron Transfer Complexes Between Reactants

This reaction profile of the polymer complex has some similarities with the phenomenon of the polyelectrolyte-catalyzed reactions. It has been reported that the reactions between two positively charged species in aqueous solution are drastically accelerated in the presence of polyanionsS2 84 For example, the electron-transfer reaction between [Co(IIIXen)2(Py)Cl]2+ and [Fe(IIXOH2)6]2+ is very slow because the reaction occurs between two cations however, the addition of a small amount of poly(styrenesulfonate) accelerates the reaction by a factor of 103 84). This result is also interpreted as indicating that the two positively charged reactants are both concentrated in the polyanion domain, so that they encounter each other more frequently [Fig. 17(b)]. 

Nonetheless, in Table 4 are summarized the results of a series of rate constant comparisons between calculated values using optical absorption data and experimental values.83 The experimental data are derived from self-exchange rate constants for couples which are structurally related to the mixed-valence dimers shown in the table. The experimental values cited are calculated values for electron transfer within the association complex between reactants as estimated from k kQ >JKA. values were calculated using equation (33) and it was assumed in the calculations that vet = 5 x 1012 s-1. Calculations of this kind have been extended to unsymmetrical dimers like (NHJ)5RuIII(pz)RuIICl(bipy)24+ 83 and even to outer-sphere ion-pairs like (4).88... 

The Marcus theory [5-7] has been mostly applied to reversible electron transfer reactions between metal complexes ([ML]"+ and [M L ]m + 1), where an electron transfer results in the formation of stable products that do not undergo further chemical reactions other than back electron transfer to regenerate reactant pairs [8-12] ... 

Fig. 10. Solute-solvent cage charge-transfer complex model for an electron-transfer reaction between a redox reactant pair donor D and acceptor A...

A vast literature exists on the kinetics and mechanisms of electron-transfer reactions between dissolved metal ions. A recent review was written by Sutin (1986). Physical chemists, however, have dealt so far almost exclusively with reactions in aqueous solution of very low pH or high ligand concentrations. Such studies have shown that the homogeneous electron-transfer between couples such as Fe(III)/Fe(II) proceeds via three distinct steps. First the two reactants diffuse together and form a reactive intermediate called the precursor complex. The electron transfer occurs after an appropriate reorganization of the nuclear configuration. This yields a short-lived product called the successor complex. Finally the successor decomposes to the separated products of the redox reaction ... 

An outer sphere mechanism involves electron transfer from reductant to oxidant with the inner coordination spheres of each remaining intact. That is, one reactant becomes involved in the outer or second coordination sphere of the other reactant, and an electron flows from reductant to oxidant. Such a mechanism is established when rapid electron transfer occurs between two substitution inert complexes. A typical example of this type of process is the reaction between [FeCCNj ] and [IrCy ... 

The solvation of reactants and of the active complex by solvent molecules affects the rates of reactions considerably. Solvation energy can be large, and make possible the occurrence of stable charged entities in solvents of high dielectric constant, where they can approach each other closely with corresponding activation energies still low. An example is the electron transfer reaction between aquo complexes of Fe and Fe " (isotopic)... 

In a complexation reaction, a Lewis base donates a pair of electrons to a Lewis acid. In an oxidation-reduction reaction, also known as a redox reaction, electrons are not shared, but are transferred from one reactant to another. As a result of this electron transfer, some of the elements involved in the reaction undergo a change in oxidation state. Those species experiencing an increase in their oxidation state are oxidized, while those experiencing a decrease in their oxidation state are reduced, for example, in the following redox reaction between fe + and oxalic acid, H2C2O4, iron is reduced since its oxidation state changes from -1-3 to +2. 

Only in a limited number of instances will the value of k and its associated parameters be useful in diagnosing mechanism. When the redox rate is faster than substitution within either reactant, we can be fairly certain that an outer-sphere mechanism holds. This is the case with Fe + and RuCP+ oxidation of V(II) and with rapid electron transfer between inert partners. On the other hand, when the activation parameters for substitution and redox reactions of one of the reactants are similar, an inner-sphere redox reaction, controlled by replacement, is highly likely. This appears to be the case with the oxidation by a number of Co(III) complexes of V(II), confirmed in some instanees by the appearance of the requisite V(III) complex, e.g. 

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