Outer-sphere process

In the case of other systems in which one or both of the reactants is labile, no such generalization can be made. The rates of these reactions are uninformative, and rate constants for outer-sphere reactions range from 10 to 10 sec b No information about mechanism is directly obtained from the rate constant or the rate equation. If the reaction involves two inert centers, and there is no evidence for the transfer of ligands in the redox reaction, it is probably an outer-sphere process. 

As regards intimate mechanism, electron transfer reactions of metal complexes are of two basic types. These have become known as outer-sphere and inner-sphere (see Chapter 4, Volume 2). In principle, an outer-sphere process occurs with substitution-inert reactants whose coordination shells remain intact in... 

Comparison of equations (2.11) and (2.15) reveals q and r to be kikilk i and A 2//r i, respectively. This enables k to be calculated from qjr. In its simplest forms the structure of the reactive intermediate can be viewed as V(OH)Cr " (when n is 1) or as VOCr (when n is 2). Similar species which have been characterized or implied kinetically are CrOCr (ref. 33), Np02Cr (ref. 37), U02Cr (ref. 31), VOV " (ref. 34), U0Pu02 + (ref. 41), Pu02pe + (ref. 42) and FeOFe + (ref. 38). Predictions on the rate of the V(III)- -Cr(lI) system, based upon Marcus theory", have been made by Dulz and Sutin on the assumption that an outer-sphere process applies. The value arrived at by these authors is 60 times lower than the experimental value. 

Outer-Sphere Electron Transfer The minimal interpenetration of the coordination spheres of the reactants is inherent in any mechanistic formulation of the outer-sphere process for electron transfer. As such, steric effects provide a basic experimental criterion to establish this mechanism. Therefore we wish to employ the series of structurally related donors possessing the finely graded steric and polar properties described in the foregoing section for the study of both homogeneous and heterogeneous processes for electron transfer. 

The same steric dichotomy persists in the oxidation of alkyl radicals by tris-phenanthrolineiron(III), in which two competing processes occur -- viz., the oxidation to alkyl cations, previously identified as an outer-sphere process in eq 3,... 

For the series of -branched alkyl radicals, the second-order rate constant in eq 3 is relatively unaffected by steric effects [compare Figure 2 (right)] as expected for an outer-sphere process. In strong contrast, the rate constant kL for ligand substitution in eq 21 is adversely affected by increasing steric effects, as shown in Figure 17. 

There are three points of significance of this result. One is that it provides strong support for the 10-step mechanism originally proposed for reaction 1. Another is that it facilitates a more robust fitting of the mechanism to the kinetic data obtained for that reaction. Thirdly, it confirms that reaction 2 has a rate constant that is four orders of magnitude greater than predicted by Marcus theory. It is concluded that reaction 2 is poorly modeled as an outer-sphere process and is better described as... 

Chemical reactivity of unfunctionalized organosilicon compounds, the tetraalkylsilanes, are generally very low. There has been virtually no method for the selective transformation of unfunctionalized tetraalkylsilanes into other compounds under mild conditions. The electrochemical reactivity of tetraalkylsilanes is also very low. Kochi et al. have reported the oxidation potentials of tetraalkyl group-14-metal compounds determined by cyclic voltammetry [2]. The oxidation potential (Ep) increases in the order of Pb < Sn < Ge < Si as shown in Table 1. The order of the oxidation potential is the same as that of the ionization potentials and the steric effect of the alkyl group is very small. Therefore, the electron transfer is suggested as proceeding by an outer-sphere process. However, it seems to be difficult to oxidize tetraalkylsilanes electro-chemically in a practical sense because the oxidation potentials are outside the electrochemical windows of the usual supporting electrolyte/solvent systems (>2.5 V). 

First, in considering the question of whether or not in a particular case we do have an outer-sphere process, we need to know accurately such quantities as bond distances. I have brought some data (shortly to be published) from Dr. J. K. Beattie s laboratory in Sydney, which show that in the 3+ 2+... 

One final point should be noted. Theoretical discussions of electron transfer processes have focused almost entirely on outer-sphere processes. When we have an inner-sphere mechanism, or sufficient electronic interaction in a dynamically trapped mixed-valence complex to produce a large separation between upper and lower potential surfaces, the usual weak-interaction approach has to be abandoned. Thus a detailed knowledge of a potential surface which is not describable as an intersection surface of perturbed harmonic surfaces, for example, is required. For this purpose, detailed calculations will be required. The theory of these processes will be linked more... 

Reverting to outer-sphere processes, one should bear in mind that the observed spread of reaction rates for these is very large iiideed—in fact, it is on the order of magnitude of the age of the universe in seconds So I think that Dr. Sutin is doing very well in explaining it. 

DR. HUSH For myself, if I gave the impression of any feeling of self-satisfaction, it was certainly in the sense that I think we can say that there is a glimmer of light in the darkness. And in chemistry we must be grateful for the smallest spark. I have stressed also that our present theoretical approaches to rate calculations have been almost entirely confined to outer-sphere processes, and that the task of formulating reliable theories for inner-sphere transfers will be a formidable one. 

An [H + ] term in the rate law for reactions involving an aqua redox partner strongly suggests the participation of an hydroxo species and the operation of an inner-sphere redox reaction (Sec. 5.5(a)). Methods (a) and (b) are direct ones for characterizing inner-sphere processes, analyzing for products or intermediates which are kinetically-controlled. Method (c) is indirect. Other methods of distinguishing between the two basic mechanisms are also necessarily indirect. They are based on patterns of reactivity, often constructed from data for authentic inner-sphere and outer-sphere processes. They will be discussed in a later section. 

The oxidation of V(II) by a large number of Co(III) complexes has been studied (Tables 5.2, and 5.7). Some oxidations are clearly outer-sphere and others inner-sphere (controlled by substitution in V(II)), and several are difficult to assign (Table 5.2). In general /SH values are much lower for outer-sphere than inner-sphere redox reactions and outer-sphere processes usually give LFER in reactions with Co(NH3)5X, Ref. 26. 

In general, the larger /Th/ d value corresponds to an outer-sphere process or an inner-sphere process which is not substitution controlled. 

Another interesting point is the relative rates of the reactions of the azido and thiocyanatopentaammines. The relative rates of these two reactants with iron (I I) ion are similar to those with chromium (I I), that is, the azide is four to five powers of ten more rapid than is the thiocyanato. I am suggesting that this might be a criterion for inner sphere activated complex as opposed to an outer sphere complex. With trisdipyridylchromium(II) ion, which must react via an outer sphere process, the azido and thiocyanato rates are relatively comparable, and the same also for vanadium (I I) ion which also probably procedes via an outer sphere activated complex. 

For the unusual reactivity of ferrocenylsilanes toward 5u in THF, affording ketones instead of the expected tertiary alcohols, a mechanism was proposed including the inner-sphere electron transfer from 5u within a reactant complex. The proposition was based on an electrochemical CV examination, which indicated that the outer-sphere process is thermodynamically unfavorable. 

According to the Marcus model, the standard rate constant, ks, for reaction (4.9) can be expressed as follows if the reaction is an adiabatic outer-sphere process 6 ... 

Redox reactions involving the nickel(IV) complex are also subject to divalent metal ion catalysis (170, 171). Oxidations of the two-electron reductant ascorbate (40) and the one-electron reductant [Fe(CN)6]4-(172) have been examined in some detail. Both reactions have as the rate-determining step the transfer of one electron from the reductant to nickel(IV) in an outer-sphere process to give an undetected nickel(III) transient. Spectroscopic properties of the nickel(III) species have been determined by pulse radiolysis (41). 

Examples of photoredox processes belonging to each of the above groups are given in Table 4. The first three types of photoredox processes (a, b, c) are mostly intracomplex monomolecular reactions (photoredox additions are bimolecular processes) which usually do not occur when a complex is in its ground state. Photoinduced long-range electron transfer reactions can be understood as a boundary between outer-sphere processes (due to a great distance between the reaction sites) and inner-sphere ones (they are, in fact, realized in one molecule). 

Further work by Flowers examined the role of solvent polarity in the electron transfer process.30 Inner-sphere electron transfer kinetics show a weak dependence on solvent polarity due to the considerable orbital overlap of the donor-acceptor pair in the transition state. In an outer-sphere process, changes in solvent polarity alter the energetics of electron transfer. The addition of excess HMPA, beyond that required to saturate Sml2, resulted in a linear correlation to the rate of reduction for alkyl iodides, whereas no impact was observed on the rate of ketone reduction.30 Thus the experiments showed a striking difference in the electron transfer mechanism for the substrate classes, which is consistent with the operation of an outer-sphere-type process for the reduction of alkyl iodides and an inner-sphere-type mechanism for the reduction of ketones.30 These findings are consistent with the observations of Daasbjerg and Skrydstrup.28,29... 

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