Sphere Mechanisms

As we have just discussed, the coordination spheres of both reactants remain intact during an outer-sphere electron-transfer reaction. The same is not true, however, of the inner-sphere reactions we now start to consider. Inner-sphere electron-transfer reactions involve the formation of a bridged complex in which the two metal ions are 

The first and now classic set of reactions involving inner-sphere electron-transfer mechanisms was reported in 1953 byTaube and his group. The overall reaction is given in Equation (5.43)  

Note that Co is reduced to Co , while Cr is oxidized to Cr. Abridging ligand (X ) is transferred from the cobalt coordination sphere to that of the chromium. 

Comparative Rate Constants for the Inner-Sphere Electron-Transfer Reaction 

Source Data from F. Basolo and R. G. Pearson, Mechanisms of Inorganic Reactions, a Study of Metal Complexes in Solution, 2d ed. (New York Wiley, 1968), 481. 

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Relationships connecting stmcture and properties of primary alkylamines of normal stmcture C, -C gin chloroform and other solvents with their ability to extract Rh(III) and Ru(III) HCA from chloride solutions have been studied. The out-sphere mechanism of extraction and composition of extracted associates has been ascertained by UV-VIS-, IR-, and H-NMR spectroscopy, saturation method, and analysis of organic phase. Tertiary alkylamines i.e. tri-n-octylamine, tribenzylamine do not extract Ru(III) and Rh(III) HCA. The decrease of radical volume of tertiary alkylamines by changing of two alkyl radicals to methyl make it possible to diminish steric effects and to use tertiary alkylamines with different radicals such as dimethyl-n-dodecylamine which has not been used previously for the extraction of Rh(III), Ru(III) HCA with localized charge. 

The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

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. 

On the basis of these results it seems to the present author that inner and outer complexes can reasonably be assumed for the electron transfer to the diazonium ion, but that an outer-sphere mechanism is more likely for metal complexes with a completely saturated coordination sphere of relatively high stability, such as Fe(CN) (Bagal et al., 1974) or ferrocene (Doyle et al., 1987 a). Romming and Waerstad (1965) isolated the complex obtained from a Sandmeyer reaction of benzenediazonium ions and [Cu B ]- ions. The X-ray structural data for this complex also indicate an outer-sphere complex. 

The Marcus treatment applies to both inorganic and organic reactions, and has been particularly useful for ET reactions between metal complexes that adopt the outer-sphere mechanism. Because the coordination spheres of both participants remain intact in the transition state and products, the assumptions of the model are most often satisfied. To illustrate the treatment we shall consider a family of reactions involving partners with known EE rate constants. 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Figure 9-4. The outer-sphere mechanism for an electron transfer reaction between two complexes. No covalently-linked intermediate is involved in the reaction.

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... 

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... 

Endicott and Taube consider that there is cause for doubt over the generally-held views that Cr(bipy)3 is oxidised by an outer-sphere mechanism"". They suggest that, since the complex is very labile to substitution, coordination sites... 

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... 

The aquation of [IrCl6]2- to [Er( E120)C1S] and Ir(H20)2Cl4 has been found to activate the complex toward the oxidation of insulin in acidic solutions, with measured rate constant of 25,900 and 8,400 Lmol-1 s 1, respectively.50 The oxidation reaction proceeds via an outer-sphere mechanism. 

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... 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

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