Mechanisms inner-sphere mechanism

Another classification of C-H activation methods is as inner-sphere and outer-sphere mechanisms. Inner-sphere mechanisms can be defined as those that involve the formation of a carbon-metal bond from a C-H bond, while outer-sphere mechanisms involve the cleavage of a C-H bond by a metal-containing species to generate a reactive intermediate, but without a metal-carbon bond. A disadvantage of this classification is that it assumes that the mechanism is known The reactions discussed in this chapter would be considered inner-sphere. Reactions such as the Fenton reaction would be considered outer-sphere. A grey area is likely to exist between the two mechanisms. Another disadvantage of this classification is that the term inner-sphere mechanism tells us nothing about the mechanism beyond the formation of a metal-carbon bond ... 

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 second mechanism involves the formation of a covalent bridge through which the electron is passed in the electron transfer process. This is known as the inner-sphere mechanism (Fig. 9-5). 

The inner-sphere mechanism is restricted to those complexes containing at least one ligand which can bridge between two metal centers. The commonest examples of such ligands are the halides, hydroxy or oxo groups, amido groups, thiocyanate... 

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

It is thought that exchange can occur through the species Fe(DMSO) and Fe(DMSO) possibly via an inner sphere mechanism the exchange occurs in the absence of water. 

S.SxlO l.mole .sec and 2xl0 1. mole sec , respectively) (c) the Cr(II)-catalysed isomerisation of CrSCN produced in (a) (k = 42 l.mole . sec ). Rate coefficients pertain to 1 M FICIO4 solutions at 25 °C. Thus an inner-sphere mechanism is demonstrated. The S-bonded thiocyanato complex, CrSCN, is not produced when a solution of Cr -FSCN is oxidised by Fe(III). CrSCN can be prepared by the gradual addition of a 5 x 10 M solution to an equal volume of a well-stirred solution of 5.5 x 10 A/ Fe([II) and 4.5 x 10 M SCN . The product solution is green whereas CrNCS solutions are purple. 

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... 

Boron is adsorbed on Fe/Al oxides (goethite and gibbsite) via an inner-sphere mechanism with shifts in zero point of charge (Goldberg et al., 1993). Boron adsorption on Fe/Al oxides increases from pH 3 to 6,... 

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... 

Traditionally, electron transfer processes in solution and at surfaces have been classified into outer-sphere and inner-sphere mechanisms (1). However, the experimental basis for the quantitative distinction between these mechanisms is not completely clear, especially when electron transfer is not accompanied by either atom or ligand transfer (i.e., the bridged activated complex). We wish to describe how the advantage of using organometals and alkyl radicals as electron donors accrues from the wide structural variations in their donor abilities and steric properties which can be achieved as a result of branching the alkyl moiety at either the a- or g-carbon centers. 

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. 

J.K. Kochi I agree. The quantitative treatment of inner-sphere mechanisms is difficult from a purely theoretical point of view. The phenomemological approach describes the activation barrier for inner-sphere process quantitatively, but provides no theoretical basis, unfortunately. 

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... 

An inner-sphere mechanism can be offered as an alternative interpretation of the kinetics for NO reduction of aqueous Cu(dmp)2(H20)2+. 

There are two mechanisms that could explain the catalytic effects of nitrite an inner-sphere mechanism in which nitrite acts as a nucleophile toward the FeIINO+ moiety (Scheme 3, pathway A) and an outer-sphere path in which nitrite is oxidized to N02 which then reacts with excess NO to form N203 (Scheme 3, pathway B). Although the initial electron transfer step in pathway B is thermodynamically uphill (AE = — 0.3 V) (41,70), one cannot rule out pathway B since N203 is rapidly hydrolyzed, once formed (71). 

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). 

The nitric oxide reduction of Cu(dmp)2(H20)2+ in aqueous media gives a Cu(II)-NO complex via an inner-sphere mechanism [216] (dmp = 2,9-dimethyl-l,10-phen-... 

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... 

According to Taube, the inner sphere mechanism can takes place when both oxidizing and reducing agents are substitution inert and when ligand transfer from oxidant to reductant is accompanied by electron transfer. The inner sphere electron transfer mechanism may be represented by the scheme... 

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 ... 

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... 

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