Electron transfer successor complex

Inner-sphere. Here, the two reactants first form a bridged complex (precursor)- intramolecular electron transfer then yields the successor which in turn dissociates to give the products. The first demonstration of this was provided by H. Taube. He examined the oxidation of ICrfHoOijj by lCoCl(NHr)< and postulated that it occurs as follows ... 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

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... 

Figure 1. Potential energy plot of the reactants (precursor complex) and products (successor complex) as a function of nuclear configuration Eth is the barrier for the thermal electron transfer, Eop is the energy for the light-induced electron transfer, and 2HAB is equal to the splitting at the intersection of the surfaces, where HAB is the electronic coupling matrix element. Note that HAB << Eth in the classical model. The circles indicate the relative nuclear configurations of the two reactants of charges +2 and +5 in the precursor complex, optically excited precursor complex, activated complex, and successor complex.

The standard formalisms for describing ET processes assume that in reactions such as Eqs. (1) and (2) there is but a single stable conformational form for each of the precursor and successor electron-transfer states. However, for a system that displays two (or more) alternative stable conformations with different ET rates, dynamic conformational equilibrium can modulate the ET rates. Major protein conformational changes can occur at rates that are competitive with observed rates of ET [9], and such gating [10] may occur in non-rigid complexes such as that between zinc cytochrome c peroxidase (ZnCcP) and cytochrome c (see below) or even within cytochrome c [5]. 

Electron transfer was considered to occur at the intersection region S of the potential energy hypersurface and precursor (before electron transfer) and successor (after electron transfer) complexes, respectively. Both energy surfaces were evaluated using the potential function that was built up with an ab initio method. For each configuration, the parameter A = - Hpp was calculated. This parameter was used as... 

The product of intramolecular electron transfer within the precursor complex is the successor complex... 

Of course the Co CNHj) breaks down rapidly in acid into Co + and 5NHJ. Precursor complex formation, intramolecular electron transfer, or successor complex dissociation may severally be rate limiting. The associated reaction profiles are shown in Fig. 5.1. A variety of rate laws can arise from different rate-determining steps. A second-order rate law is common, but the second-order rate constant is probably composite. For example, (Fig. 5.1 (b)) if the observed redox rate constant is less than the substitution rate constant, as it is for many reactions of Cr +, Eu +, Cu+, Fe + and other ions, and if little precursor complex is formed, then = k k2kz ). In addition, the breakdown of the successor complex would have to be rapid k > k 2). This situation may even give rise to negative (= A//° +... 

A ] represent the reorganized precursor and successor complexes involved in the electron transfer step. This scheme predicts that the observed activation energy will switch from a positive to a negative value if the relaxation of [D/A] back to [D/ A] has a larger temperature dependence than the reorganization of [D/A] to [D/A]. In the... 

From Eq. 14-30 we see that we may divide a one-electron transfer into various steps (maybe somewhat artificially). First, a precursor complex (PR) has to be formed that is, the reactants have to meet and interact. Hence, electronic as well as steric factors determine the rate and extent at which this precursor complex formation occurs. Furthermore, in many cases, redox reactions take place at surfaces, and therefore, the sorption behavior of the compound may also be important for determining the rate of transformation. In the next step, the actual electron transfer between P and R occurs. The activation energy required to allow this electron transfer to happen depends strongly on the willingness of the two reactants to lose and gain, respectively, an electron. Finally, in the last steps of reaction sequence Eq. 14-30, a successor complex may be postulated which decays into the products. 

For an inner-sphere reaction there are necessarily more steps since both association and substitution must precede electron transfer. Intermediates like (H20)5CruClCoUI(NH3)54+ and (H20)5CrinClCoII(NH3)54 shown in Scheme 2 are often referred to as the precursor and successor complexes since they precede or follow the electron transfer step. 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

It should also be pointed out that the rate of each of the reaction steps (precursor complex formation, electron transfer, and breakdown of successor complex) is affected by the chemical characteristics of the metal oxide surface sites and the nature of the reductant molecules. These aspects are discussed in detail in an excellent review by Stone (1986), and the reader is encouraged to refer to this article. 

The first step involves the formation of the precursor complex, where the reactants maintain their identity. In the second step there is, as we will see later, reorganization of the inner coordination shells as well as of the solvation spheres of the reactants so as to obtain a nuclear configuration appropriate to the activated complex through which the precursor complex is transformed into the successor complex. The electron transfer usually occurs during the latter stages of this reorganization process. The activated complex deactivates to form the successor complex if electron transfer has occurred or to reform the precursor complex if electron transfer has not occurred. The electron distribution in the successor complex corresponds to that of the products, so that the third step is simply the dissociation of the successor complex to form the separated products. 

Because the electronic distribution and nuclear configuration of the donor and the acceptor in the (Class II) successor complexes are similar to those of the free donor/acceptor product (i.e. radical pair), it is reasonable to suggest that products can originate directly from the successor complex (pathway Pi). Such a reaction, which includes an electron-transfer step, does not necessarily proceed via a pair of free ion radicals, and the effective activation energy can be even lower than that required by pathway P2. When the follow-up reaction involves the coupling of radicals, the reaction directly proceeding from the (ET) successor complex state can be kinetically favorable (since it excludes diffusional processes). 

This value is much larger than that observed in the previously reported deracemizations of [Cr(ox)3]3 and Cr(acac)3. In this reaction, the basic condition is necessary, and the addition of Hacac increases the enantiomer excess, for which the reason will be discussed below. The reaction mechanism shown in Scheme 17 was proposed. In the mechanism, the 3MLCT excited A- [Ru(( — )-men-bpy)3]2+ is oxidatively quenched by Co(acac)3 to form an exciplex with Co(acac)3 followed by electron transfer to Co(acac)3 from A- [Ru(( — )-menbpy)3]2 +, which leads to the formation of a successor complex, [A-Ruin(( — )-menbpy)33 + Con(acac)3 ]. This successor complex dissociates to A-[Rum(( — )-menbpy)3]3+, Co(acac)2, and acac. If the reducing reagent is absent or the reducing reagent does not effectively reduce the ruthenium(III) complex, Co(acac)2 reduces A-[Rum(( — )-menbpy)3]3+ to A-[Run(( — )-menbpy)3]2+ concomitantly with the formation of Co(acac)3. As discussed in Sec. II.A., the photoreduction of Co-(acac)3 occurs stereoselectively. In addition, the oxidation of Co(acac)2 to Co-(acac)3 occurs stereoselectively, because Co(acac)2 reacts with the chiral ruthen-... 

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