Outer-sphere electron self-exchange reaction

The Marcus therory provides an appropriate formalism for calculating the rate constant of an outer-sphere redox reaction from a set of nonkinetic parameters1139"1425. The simplest possible process is a self-exchange reaction, where AG = 0. In an outer-sphere electron self-exchange reaction the electron is transferred within the precursor complex (Eq. 10.4). 

The rate constant and activation parameters for the outer-sphere electron self-exchange reaction of the [Ni(C5H5)2] couple have been measured in dichloromethane by means of H NMR line-shape analysis. The very rapid exchange process, = 2.2 x 10 s (a value of 1 x 10 Af s is... 

On the other hand, [Cu(I)L ]" (10,24), a stronger reducing agent than Cu" (aq), affects the rate constants of the reduction reactions in a more complex way. The rate constant of the outer-sphere reduction is nearly not affected, that is, the decrease in the rate constant of the electron self-exchange reaction compensates the increase in the redox potential. For the inner-sphere reactions, slows down the reaction with ClsCCOO" probably due to steric hindrance. On the other hand, accelerates considerably the reduction of NO2 probably by stabilizing the transient complex L Cu(I)N02 and has only a minor effect on the rate of reduction of [Co(III)(NH3)5Cl]. ... 

A mechanism involving the polarization of the ascorbate ligand by a Cu(II) central ion was proposed (138), though the involvement of Cu(I) cannot be ruled out (139). All these reactions proceed via the inner-sphere mechanism however, the copper-catalyzed reduction of superoxide boimd to a binuclear cobalt(III) complex by 2-aminoethanethiol proceeds via the outer-sphere mechanism (140). This is attributed to the effect of 2-aminoethanethiol as a hgand on the rate constant of the Cu(ll/1) electron self-exchange reaction which is suggested to proceed via the gated mechanism. 

Electron self-exchange reaction between O2 and 02 was then discussed, and developments before and after an experimentally determined rate constant for this reaction was published, were also summarized. Related to this, the problem of size differences between O2 or 02 and their typical metal-complex electron donors or acceptors was recently solved quantitatively by addition of a single experimentally accessible parameter, A, which corrected the outer-sphere reorganization energy used in the Marcus cross relation. When this was done, it was found that rate constants for one electron oxidations of the superoxide radical anion, 02 , by typical outer-sphere oxidants are successfiiUy described by the Marcus model for adiabatic outer-sphere electron transfer. 

A powerful application of outer-sphere electron transfer theory relates the ET rate between D and A to the rates of self exchange for the individual species. Self-exchange rates correspond to electron transfer in D/D (/cjj) and A/A (/c22)- These rates are related through the cross-relation to the D/A electron transfer reaction by the expression... 

Outer-sphere electron transfer reactions involving the [Co(NH3)6]3+/2+ couple have been thoroughly studied. A corrected [Co(NH3)6]3+/2+ self-exchange electron transfer rate (8 x 10-6M-1s-1 for the triflate salt) has also been reported,588 which is considerably faster than an earlier report. A variety of [Co(NH3)g]3+/2+ electron transfer cross reactions with simple coordination compounds,589 organic radicals,590,591 metalloproteins,592 and positronium particles (electron/ positron pairs)593 as redox partners have been reported. 

The theoretical results obtained for outer-sphere electron transfer based on self-exchange reactions provide the essential background for discussing the interplay between theory and experiment in a variety of electron transfer processes. The next topic considered is outer-sphere electron transfer for net reactions where AG O and application of the Marcus cross reaction equation for correlating experimental data. A consideration of reactions for which AG is highly favorable leads to some peculiar features and the concept of electron transfer in the inverted region and, also, excited state decay. 

The simplest outer-sphere electron transfer reaction is the so-called self-exchange reaction, where the two reactants are converted one into the other by electron transfer. For example ... 

Several studies of bimetallic complexes in which the donor and acceptor are linked across aliphatic chains have demonstrated that these are generally weakly coupled systems. " Studies of complexes linked by l,2-bis(2,2 bipyridyl-4-yl)ethane (bb see Figure 5), indicate that these are good models of the precursor complexes for outer-sphere electron-transfer reactions of tris-bipyridyl complexes. A careful comparison of kinetic and spectroscopic data with computational studies has led to an estimate of //rp = 20cm for the [Fe(bb)3pe] + self-exchange electron transfer. In a related cross-reaction, the Ru/bpy MLCT excited state of [(bpy)2Ru(bb)Co(bpy)2] + is efficiently quenched by electron transfer to the cobalt center in several resolved steps, equations (57) and (58). ... 

Another useful linear relationship is based on electrochemical data and is obtained by recourse to the fact that AG° = —nFE°. For a series of outer-sphere electron transfer reactions that meet the criteria discussed in context with Equation 1.14, a plot of In k versus E° will have a slope of 0.5(nF/RT), and a plot of log k versus E° will have a slope of 0.5(nF)/2.303RTor 8.5 V-1 for n = 1 at 25°C.5 All the above methods can be used to obtain a common (approximate) value of X for a series of similar reactions. For single reactions of interest, however, X values can often be measured directly by electron self-exchange. 

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