Reaction inner sphere mechanism

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. 

Figure 9-5. The inner-sphere mechanism for an electron transfer reaction between two complexes. A covalently-linked intermediate is involved in this reaction.

Candlin and Halpern comment that the sequence of rapid rates observed for Cr " " as a reductant i.e. Co(NH3)5p > Co(NH3)sBr > Co(NH3)5CI > Co(NH3)5F ) is contrary to that found for the slow reactions of Fe (ref. 126) and Eu (ref. 113). All three reductants would appear to favour inner-sphere mechanisms, but in the case of Fe and Eu the order of reactivity seems to be connected with the stability of the product halide complex (FeX or EuX ) which increases in the order X = 1 to X = F . Or in other words, as pointed out by Halpern and Rabani, in the generalised inner-sphere reaction... 

Espenson has shown that the reaction of c/j-Co(en)2(N3)2 with takes place by an inner-sphere mechanism. This Co(III) complex was selected for investigation because it is particularly reactive towards and also the dissociation of monoazido vanadium(lll) is relatively slow. At low concentrations (2-20 X 10 M) the second-order rate coefficient is 32.9 l.mole . sec at 25 °C, [H ] = 0.10 M and [i = 1.0 M. At higher concentrations ( 0.1 M), using a stopped-flow apparatus, the kinetics are apparently first order at 520 mfi, a wavelength where shows negligible absorbance. The rate coefficient under... 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

A principal distinction is made between outer-sphere and inner-sphere mechanisms in ET reactions (Kochi, 1988). In the outer-sphere reactions the... 

The prospects for electron transfer mechanisms clearly extend beyond inorganic chemistry into the broad regions of organometal-lic and organic systems. Pushed to these limits, adequate quantitative criteria will be needed to delineate outer-sphere from inner-sphere mechanisms. However, the extent to which theoretical studies will provide more concrete guidelines of predictive value will determine whether electron transfer processes will form the basis of reaction mechanisms into the next century. 

The kinetics and the mechanism of superoxide reduction by SORs have been studied by several researchers. It was suggested that SORs react with superoxide via an inner-sphere mechanism, binding superoxide at ferrous center to form a ferric hydroperoxo intermediate [46,48 50]. The rate constant for this reaction is equal to 108 109 1 mol-1 s-1 [46,49], This... 

It is thus reasonable to anticipate that HOC1 could behave as an outer-sphere one-electron oxidant. Indeed, the standard potential for the HOC1/HOC1 - couple is estimated at 0.25 V (9). In prior reports where such a pathway might have been uncovered, alternative pathways generally have been found, such as inner-sphere mechanisms and reactions via Cl2. Reaction via Cl2 is often a viable pathway because of the presence of Cl- either as a contaminant or reaction product and its reaction with HOC1 as in Eq. (4). 

The common element of Schemes 1-3 is that they each postulate direct interaction between the metal center and dioxygen. Although it is not stated explicitly, Eqs. (3) and (11) most likely proceed via an inner-sphere mechanism. Thus, the metal-dioxygen interaction implies spin pairing between the reactants when the metal ion is paramagnetic. As a consequence, the formation of the M-O2 type intermediates circumvents the restriction posed by the triplet to singlet transition which seems to be the major kinetic barrier of autoxidation reactions (5). 

It has in general been the objective of many mechanistic studies dealing with inorganic electron-transfer reactions to distinguish between outer- and inner-sphere mechanisms. Along these lines high-pressure kinetic methods and the construction of reaction volume profiles have also been employed to contribute toward a better understanding of the intimate mechanisms involved in such processes. The differentiation between outer- and inner-sphere mechanisms depends... 

Electron transfer reactions may follow two types of mechanism (i) outer sphere mechanism and (ii) inner sphere mechanism. 

The kinetic behavior of the reductive dissolution mechanisms given in Figure 2 can be found by applying the Principle of Mass Action to the elementary reaction steps. The rate expression for precursor complex formation via an inner-sphere mechanism is given by ... 

This rules out, I would think completely, a dominant outer-sphere mechanism for that system, because the observed rate is just too fast to be compatible with this. The self-exchange reaction must almost certainly proceed most favourably via an inner-sphere mechanism. More data of this kind are evidently needed. 

In this picture, the electron transfer processes mediated by metallic electrodes (redox reactions in a heterogeneous phase) can also be classified to proceed according to outer-sphere or inner-sphere mechanisms (obviously, considering the electrode surface as a reagent). 

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

Chromiain(ii) Complexes.—The oxidation of chromium(ii) in alkaline solution has been studied polarographically and the reaction shown to be irreversible with = — 1.65 V vs. S.C.E. In the presence of nitrilotriacetic acid, salicylate, ethylenediamine, and edta the values were determined as —1.075, —1.33, — 1.38, and —1.48 V, respectively. The production of [Cr(edta)NO] from [Cr (edta)H20] and NO, NOJ, or NO2 suggests that this complex is able to react via an inner-sphere mechanism in its redox reactions. ... 

The hydrogenation reaction mechanisms may be classified according to the role played by the substrate in the coordination sphere of the metal catalyst. Thus, those mechanisms proceeding with coordination of the substrate to the metal center can be labeled as inner-sphere mechanisms, whereas those with no direct coordination of the substrate to the metal center can be labeled as outer-sphere reaction mechanism (see Scheme 4). Hydrogenation reactions belonging to the so-called hydrogen transfer reactions (where the hydrogen source is usually an alcohol) can be also classified within these two families of reaction... 

Thus, in hydrogen-transfer reactions, most of the catalysts do prefer the outer-sphere mechanism instead of the MPV or the insertion mechanisms. For instance, the high stability of the intermediate formed, alkoxide in the case of carbonyl hydrogenation, is a major drawback for the inner-sphere mechanism. Nevertheless, in some particular cases, the inner-sphere mechanism may be competitive with the outer-sphere one. In these cases, some requirements must be accomplished, such as the high lability of one of the metal ligands in order to allow easily the substrate coordination or the formation of not very stable intermediates. 

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

The superoxo-containing species [(NC)6Co(/u.-02)Co(CN5]5 can be reduced with thiols such as 2-aminoethanethiol or L-cysteine (175), and the reduction reaction is catalyzed by copper(II) ions in aqueous solution. When copper(II) is present, the role of the thiol is to reduce cop-per(II) to copper(I), which then reacts with the superoxo species through an inner-sphere mechanism. Conversely, when the superoxo complex [(H3N)5Co(/x-02)Co(NH3)5]5+ is reduced with thiol (176), the reaction follows an outer-sphere mechanism, as would be expected. Ascorbic acid also reduces both complexes (177), but only the reduction of the cyano-containing complex exhibits copper(II) catalysis. 

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