Metals Outer-sphere mechanism

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. 

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. 

In aqueous solutions these reactions seem to proceed via an outer-sphere mechanism on most metals. Typically such reactions involve metal ions surrounded by inert ligands, which prevent adsorption. Note that the last example reacts via an outer-sphere pathway only if trace impurities of halide ions are carefully removed from the solution otherwise it is catalyzed by these ions. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Similarly, inner-sphere and outer-sphere mechanisms can be postulated for the reductive dissolution of metal oxide surface sites, as shown in Figure 2. Precursor complex formation, electron transfer, and breakdown of the successor complex can still be distinguished. The surface chemical reaction is unique, however, in that participating metal centers are bound within an oxide/hydroxide... 

The Electron Transfer Step. Inner-sphere and outer-sphere mechanisms of reductive dissolution are, in practice, difficult to distinguish. Rates of ligand substitution at tervalent and tetravalent metal oxide surface sites, which could be used to estimate upward limits on rates of inner-sphere reaction, are not known to any level of certainty. 

Examination of the data for (5.49) and (5.50) in Tables 5.7 and 5.8 shows that there is some general order of reactivity for the various ligands L. Containing an unshared electron pair after coordination appears a minimum requirement for a ligand to be potential bridging group, for it has to function as a Lewis base towards two metal cations. Thus CofNHj) and Co(NH3)jpy + oxidize Cr by an outer-sphere mechanism, giving Cr " as the product, at a much slower rate than for the inner-sphere reactions. 

In terms of electron transfer reactions, transition metal ions can be the one- or two-electron type. The two-electron ions transform into unstable states on unit change of the metal oxidation number. In the outer-sphere mechanism, two-electron transfer is a combination of two one-electron steps. 

The action of one-electron redox systems is readily understandable in the context of inner- and outer-sphere mechanisms, whereas two-electron redox systems require additional considerations. First, if a double one-electron transfer is possible from an organic substrate to the same metal ion, does it mean that the same molecule of an organic donor provides these two electrons, or do two molecules of the substrate act as one-electron donors ... 

The previous section described a two-step mechanism. The first step is the coordination of the substrate into the metal coordination sphere. The second is the most characteristic step within the inner-sphere mechanism the insertion of the substrate into the M-H bond. Nevertheless there are other mechanistic options that include neither substrate coordination nor M—H insertion. They are outer-sphere mechanisms and in turn can be classified as bifunctional and ionic mechanisms. 

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. 

There has been some exploration of the mechanism of reduction of d transition metal complexes by M2+(aq) (M = Eu, Yb, Sm). Both inner- and outer-sphere mechanisms are believed to operate. Thus the ready reduction of [Co(en)3]3+ by Eu2+(aq) is necessarily outer-sphere. 2 However, the strong rate dependence on the nature of X when [Co(NH3)5X]2+ or [Cr(H20)5X]2+ (X = F, Cl, Br or I) are reduced by Eu2+(aq) possibly suggests an inner-sphere mechanism.653 The more vigorous reducing agent Yb2+ reacts with [Co(NH3)6]3+ and [Co(en)3]3+ by an outer-sphere route but with [Cr(H20)5X]2+ (X = halide) by the inner-sphere mechanism.654 Outer-sphere redox reactions are catalyzed by electron-transfer catalysts such as derivatives of isonicotinic acid, one of the most efficient of which is iV-phenyl-methylisonicotinate, as the free radical intermediate does not suffer attenuation through disproportionation. Using this catalyst, the outer-sphere reaction between Eu2+(aq) and [Co(py)(NH3)5]3+ proceeds as in reactions (18) and (19). Values found were ki = 5.8 x KFM-1 s 1 and k kx = 16.655... 

As discussed in Vol. 2, Chap. 4, experimental studies, mainly pioneered by Taube [11], revealed two different reaction pathways for redox reactions in solution (i) outer sphere mechanism characterized by weak interaction of the reactive species, with the inner coordination sphere remaining intact during the electron transfer, and reactions occurring through a common ligand shared by the metallic centers thus proceeding by an inner sphere mechanism. 

Like all redox reactions26,28,967,968 those of copper(II) may be divided into two types (a) outer sphere mechanisms involving electron (or proton) transfer between coordination shells that remain essentially intact and (b) inner sphere mechanisms in which the oxidizing and reducing species are connected by a bridging ligand, which is common to both metal ion coordination spheres."9... 

Similarly to the low chemical reactivity of (simple) alkylsilanes devoid of functional groups, the electrochemical reactivity of simple alkylsilanes is quite low. Klingler and Kochi measured the oxidation potentials of tetraalkyl derivatives of group-14-metal compounds by using cyclic voltammetry3. These compounds exhibit an irreversible anodic peak in acetonitrile. The oxidation potential (7 p) decreases in the order of Si>Ge>Sn>Pb as illustrated in Table 1. This order is the same as that of the gas-phase ionization potentials (7p). The absence of steric effects on the correlation of Ev with 7p indicates that the electron transfer should take place by an outer-sphere mechanism. Since tetraalkylsilane has an extremely high oxidation potential (>2.5 V), it is generally difficult to oxidize such alkylsilanes anodically. 

Let us consider the kinetics of a one-electron transfer between a metal electrode and a molecule in solution via an outer-sphere mechanism (no strong interaction between the electroactive species and the electrode takes place see Fig. 1.12) and with no bond breaking or formation ... 

Figure 8.1. Reduction of tervalent metal oxide surface sites by phenol (HA) showing inner-sphere and outer-sphere mechanisms. [From Stone (1986), with permission.]...

It has been shown that the oxidation-reduction reactions of transition metal complexes can occur through two different mechanisms (i) inner-sphere mechanism, when the two reactants share one or more ligands of their first coordination spheres in the activated complex and (ii) outer-sphere mechanism, when the first coordination spheres of the two reactants are left intact (as far as the number and kind of ligands present are concerned) in the activated complex. As mentioned before, the inner-sphere reactions cannot be faster than ligand substitution and... 

The reaction mechanisms of Fe-SOD and Mn-SOD are not yet as well characterized as those of CuZn-SOD, but the ligands of the metals have been confirmed to be three histidyl immidazoles and one aspartyl carboxylate. The Fe- and Mn-SODs undergo alternate reduction and reoxidation during the catalytic cycle, similar to CuZn-SOD, at a rate constant of 2X109 M s l. The electron transfer to O2 from the metals proceeds through an outer sphere mechanism for Fe- and Mn-SODs, while it proceeds via an inner sphere mechanism for CuZn-SOD. Fe- and Mn-SODs exhibit substrate saturation77 as does CuZn-SOD. 

A pulse radiolysis study of Mn-SOD from B. stearothermophilus has indicated a fast outer sphere mechanism of the interaction of O2 with the metals at low O2 concentrations (5.6X109 M s 1) and a slow inner sphere mechanism at higher O2 concentrations (4.8X107 M-1 s l). The latter mechanism is becoming more important at high temperatures.78 NMR studies support an outer sphere mechanism by which O2 interacts with the Mn3+ through Mn-bound water.79 The proposed reaction for the fast cycle is ... 

Since most inorganic redox processes take place between two metal complexes, they are classified according to the behaviour of the inner (first) co-ordination spheres (shells) in the transition state (Basolo and Pearson, 1967). In the transition state of an outer-sphere mechanism the inner co-ordination spheres of both metal ions are intact, i.e., no ligand to metal bond is broken or formed, whereas in the transition state of an inner-sphere mechanism a bridging ligand,... 

Our results indicate that the autoreduction cannot occur by a conventional outer sphere mechanism because of the gross mismatch of the electrochemical potentials. Experimental data available at this time are consistent with homolytic iron-carbon bond cleavage which may or may not involve a simultaneous nucleophilic attack on the coordinated cyanide. The homolytic metal-carbon bond cleavage may serve as a model for similar processes reported for vitamin Bi2 (26). 

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