Examples of Outer-Sphere Reactions

Table 2. Examples of Outer-Sphere Reactions Involving Biologically Active Species...

One example of outer-sphere electron transfer is the reaction between the dipotassium cycloocta-tetraene (KjCgHg) and the cobalt complex of bis(salicylidenediamine) (Co Salen) (Levitin et al. 1971). 

There is a lot of examples of outer-sphere electron-transfer reactions occurring in irradiated systems of typical inorganic complexes [188-191]. However, for metallotetrapyrroles such reactions involving the central atom are rather rare. It should be underlined that it is usually not so simple to distinguish between the primary outer-sphere and inner-sphere step, particularly in cases when both lead to the same product and proceed with comparable rates. Moreover, a number of outer-sphere electron-transfer reactions occur as reversible processes with no net chemical change. To solve this problem, techniques of... 

The following reactions are examples of outer-sphere electron transfers ... 

N. Sutin, Brookhaven National Laboratory Strictly speaking, the outer-sphere and inner-sphere designations refer to limiting cases. In practice, reactions can have intermediate outer-sphere or inner-sphere character this occurs, for example, when there is extensive interpenetration of the inner-coordination shells of the two reactants. Treating this intermediate situation requires modification of the usual expressions for outer-sphere reactions — particularly those expressions that are based upon a hard-sphere model for the reactants. 

In the case of stepwise electron-transfer bond-breaking processes, the kinetics of the electron transfer can be analysed according to the Marcus-Hush theory of outer sphere electron transfer. This is a first reason why we will start by recalling the bases and main outcomes of this theory. It will also serve as a starting point for attempting to analyse inner sphere processes. Alkyl and aryl halides will serve as the main experimental examples because they are common reactants in substitution reactions and because, at the same time, a large body of rate data, both electrochemical and chemical, are available. A few additional experimental examples will also be discussed. 

From a mechanistic point of view [4], there are two extremes conceivable if the interaction between X and Y is weak, [XY] symbolizes a transition state and the reaction (la,a ) is a case of outer-sphere electron transfer. If, however, the interaction is strong enough that it leads to, e.g., covalent-bond formation between X and Y (Eq. 1 b), the product of that interaction is an intermediate ( adduct ) and the overall electron exchange between X and Y (via Eq. lb, b ) is then an example of inner-sphere electron transfer. 

The analogy between electron-transfer via addition/elimination (Eq. 2b,c) or abstraction/elimination (Eq. 2a, c) and classical solvolysis involving closed-shell molecules (nonradicals) is seen by comparing Scheme 1 with Scheme 3, in which XY, the precursor of the ions X and Y , is formally derived from the two radicals X and Y". Analogous to Scheme 1, on the way to the ionic products that result from the interaction between X and Y there are two possibilities if XY denotes a transition state, the reaction (Eq. 3a, a ) is a case of outer-sphere electron transfer. If, however, a covalent bond is formed between X and Y, the path (Eq. 3b, b ) is an example of inner- sphere electron transfer. Obviously, part b of the scheme describes the classical area of S l solvolysis reactions (assuming either X or Y to be equal to C) [9, 10]. If a second reaction partner for C (other than the solvent) is allowed for (the (partial) ions then represent transition states), then Eq. 3b also covers Sn2 reactions. If looked upon from the point of view of radical-radical reactivity, Eqs. 3a and b show well-known reactions radical disproportionation in Eq. 3a,a and combination in Eq. 3b. 

Nevertheless, the mechanism of the Shvo s catalyst has been one of the most controversial regarding the nature of the hydrogen-transfer process (84). The analysis of this reaction mechanism served as an example of comparison of both the inner- and outer-sphere reaction pathways for hydrogenation of polar, C=0 (85-87) and C=N (88—95) and unpolar bonds (95). In the next subsections are presented the mechanistic studies we carried out for the hydrogenation of ketones, imines, alkenes, and alkynes (29,87,95). 

A point of note in the data in Table 1 is the extraordinary range in electron-transfer reactivity that can exist even for outer-sphere reactions among what appear to be closely related reactions. For example, the self-exchange rate constants for Co(NH3)63+/2+ and Ru(bipy)33+/2+ differ in magnitude by 1014. 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

It is conventional to classify electrochemical reactions as outer-sphere and inner-sphere. The former involve the outer coordination sphere of a reacting ion. Thus, little if any change inside the ion solvate shell occurs they proceed without breaking-up intramolecular bonds. But in the latter, involving the inner coordination sphere, electron transfer is accompanied by breaking up or formation of such bonds. Often the inner-sphere reactions are complicated by adsorption of reactants and/or reaction products on the electrode surface. The electron transfer in the Fc(CN)62 /4 system is example of an outer-sphere reaction (with due reservation for some complications... 

The reaction of dissolved C02 with hydroxide ions depicted in Eq. 1.4 takes place entirely in the aqueous solution phase and so is termed homogeneous.1 Another example of a homogeneous reaction is the formation of an outer-sphere complex by Mn2+ and Cl" in a soil solution 4... 

This article is concerned with reactions in which the net effect is transfer of one or more electrons, with little permanent rearrangement of the structures surrounding the donor and acceptor sites. Typical examples are the following an outer-sphere reaction (equation 1), an inner-sphere reaction (equation 2) that involves direct bridging Cr-Cl-Cr in the transition state, and a two-electron reaction (equation 3) that could be classed as atom transfer, but is formally analogous to equation (2). 

The fonnulas quoted in this section have been used to estimate absolute rates of a considerable number of outer-sphere electron-transfer reactions. The reactions Rn(OH2)6 +/2+ and Ru(NH3)6 +/2+,3 and Mn04- are particnlarly well-studied examples. The dependence of rate on solvent polarity implied by eqnation (6) has proved more difficult to test and it has only recently become clear that some reactions, such as [Ru(hfac)3]°/, fit the equation well, while others, such as [Mn(NC(C6Hn)6)] + +, do not. The variation in polarity is usually achieved by using mixed solvents in varying proportions, and selective solvation by one component may be one reason for discrepancies. 

Figure 2.12 illustrates schematically the essential features of the thermodynamic formulation of ACT. If it were possible to evaluate A5 ° and A// ° from a knowledge of the properties of aqueous and surface species, the elementary bimolecular rate constant could be calculated. At present, this possibility has been realized for only a limited group of reactions, for example, certain (outer-sphere) electron transfers between ions in solution. The ACT framework finds wide use in interpreting experimental bimolecular rate constants for elementary solution reactions and for correlating, and sometimes interpolating, rate constants within families of related reactions. It is noted that a parallel development for unimolecular elementary reactions yields an expression for k analogous to equation 128, with appropriate AS °. 

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