Inner-sphere redox reactions

Demonstration of ligand transfer is crucial to the proof that this purticitlar reaction proceed.s via an inner-sphere mechanism, and ligand transfer i.s indeed a usual feature of inner-sphere redox reaction.s, but it is not an essential feature of oil such reactions. 

The most important single development in the understanding of the mechanisms of redox reactions has probably been the recognition and establishment of outer-sphere and inner-sphere processes. Outer-sphere electron transfer involves intact (although not completely undisturbed) coordination shells of the reactants. In inner-sphere redox reactions, there are marked changes in the coordination spheres of the reactants in the formation of the activated complex. 

Reaction 4 in Table 5.1, on the other hand, was one of the first-established examples of an inner-sphere redox reaction. The rapid reaction gives CrCl " as a product, characterized spectrally after separation by ion exchange from the remainder of the species in solution. It is clear that since CrCl " could not possibly be produced from Cr " and Cl ions during the brief time for reaction and ion-exchanger manipulation, it must arise from the redox pro-... 

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. 

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. 

Fig. 5.1 Reaction profiles for inner-sphere redox reactions illustrating three types of behavior (a) prercur-sor complex formation is rate-limiting (b) precursor-to-successor complex is rate-limiting and (c) breakdown of successor complex is rate-limiting. The situation (b) appears to be most commonly encountered.

The Bridging Ligand in Inner-Sphere Redox Reactions... 

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. 

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

Redox electrode reactions on metal electrodes constitute the simpler case for a theoretical approach to the problem. In particular, outer sphere redox electrode reactions not involving specific adsorption interactions have been treated successfully in analogy with homogeneous redox reactions in solution [54, 56], Approximate extension of the theoretical approach to the case of inner sphere redox reactions at electrodes has been done [56, 57b]. 

Underpotential deposition of metal adatoms at foreign metal electrodes shows a strong effect on the kinetics of inner sphere redox reactions such as the reduction of Cr(OH2)sCl2+ [130] due to electrostatic and specific interactions. 

This is a typical inner sphere redox reaction. The proton is heavily solvated (and probably exists in solution as [H9O4], while the... 

Marcus LFER. Oxidation-reduction reactions involving metal ions occur by (wo types of mechanisms inner- and outer-sphere electron transfer. In the former, the oxidant and reductant approach intimately and share a common primary hydration sphere so that the activated complex has a bridging ligand between the two metal ions (M—L—M ). Inner-sphere redox reactions thus involve bond forming and breaking processes like other group transfer and substitution rcaclions, and transition-state theory applies directly to them. In outer-sphere electron transfer, the primary hydration spheres remain intact. The... 

This is an example of inner sphere redox reaction. The reaction Chemistry Practical Application... 

We now turn to the inner-sphere redox reactions in polar solvents in which the coupling of the electron with both the inner and outher solvation shells is to be taken into account. For this purpose a two-frequency oscillator model may the simplest to use, provided the frequency shift resulting from the change of the ion charges is neglected. The "adiabatic electronic surfaces of the solvent before and after the electron transfer are then represented by two similar elliptic paraboloids described by equations (199.11), where x and y denote the coordinates of the solvent vibrations in the outer and inner spheres, respectively. The corresponding vibration frequencies and... 

In this situation the rate constant of inner-sphere redox reactions can be evaluated by applying the adiabatic formulation of reaction rate theory and by using, for instance, the basic equation (103.III) which yields (since g = 1)... 

The adiabatic inner-sphere redox reactions were first treated by MARCUS /145/, who made use of the classical and semiclassical statistical theory A quantum-mechanical treatment of the two-frequency oscillator model by DOGONADZE and KUSNETSOV /147/ provides tractable rate expressions for non-adiabatic processes in both high and low temperature ranges. Similar results were obtained by KESTNER, LOGAN and JORTNER /148/. 

For outer-sphere electron transfer, the rate equations (62.IV) and (63.IV) for non-adiabatic and adiabatic reactions may be used by introducing the electrode potential cp through the relation (107.IV) or the overvoltage through equation (114.IV), instead of the reaction heat Q. For inner-sphere redox reactions the expressions (83 IV) and (85.IV) can be used in a similar way for electronically non-adiabatic and adiabatic reactions, respectively. The conditions of validity of the Tafel equation are then given by ( 12.IV) or (116.IV). 

Platinum(Tv) Complexes. Substitution reactions at this centre are generally ligand replacements, but with the added feature of marked catalysis by platinum(n) compounds. Indeed, this area is as much one of inner-sphere redox reactions as of substitution. Recent kinetic studies include those of substitution at /ra/ij-[PtCl2L4] +, where L = NHj, amine,... 

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