Metal-centered electron transfer

The electrochemical reduction of permanganic acid [H0Mnvu(0)3], which is traditionally represented as a metal-centered electron transfer to change Mn7+ to Mn6+, is another example of a ligand-centered process ... 

In the case of an outer-sphere reaction the so-called bridging material is simply the material between the redox centers—solvent molecules and, in the case of metal complexes, ligands surrounding the metal centers. Electron transfer between donor and acceptor sites connected by a molecular bridge is now fairly well understood. The rates decrease with increasing separation of the donor and acceptor and can generally be interpreted in terms of a first-order rate constant kei-... 

The coordination chemistry of (phenoxyl)manganese complexes is rather more complicated because both metal- and ligand-centered electron-transfer processes are accessible in the normal potential range. The phenolato precursors are known to exist with manganesc(II), (III), and even (IV). In fact, three phenolato groups strongly stabilize the Mn(IV) oxidation state. 

Electron transfer between metal centers can alter the course of reaction in several ways (46). Thermal excitation may create especially reactive electron holes on the oxide surface, causing reductant molecules to be consumed at the surface at a higher rate. More importantly, electrons deposited on surface sites by organic reductants may be transferred to metal centers within the bulk oxide (47). This returns the surface site to its original oxidation state, allowing further reaction with reductant molecules to occur without release of reduced metal ions. Electron transfer between metal centers may therefore cause changes in bulk oxide composition and delay the onset of dissolution. 

The examples presented in this chapter illustrate that many molecules without metals undergo redox processes in which the voltammetric current is proportional to their concentration. Often these nonmetallic substrates give responses that are due to the facilitated electron-transfer reduction of H30+/H20 or oxidation of H0 /H20. Hence, any substrate that forms a strong bond with H- or HO1 (or has an HO—/ or an R—H group with weak bonds to yield H—OH) will facilitate these electron-transfer processes at less extreme potentials to give peak currents that are proportional to the substrate concentration. The next two chapters (on organic compounds and organometallic compounds) include many more examples of matrix-centered electron-transfer redox processes. 

In most of the metal-organic dyads described in this review the metal center has a d6 electronic configuration. Further, the lowest excited state typically has a metal-to-ligand charge transfer (MLCT) configuration arising from promotion of a metal centered -electron into a ligand based -tt level, e.g.,... 

Marcus theory of electron transfer (Eq. 4) [91] to the rate of electron transfer from ferrocyanide to HRP compound I (8 x 10 M s ) [105], An even larger reorganization energy (2 = 78.0 kcal mol ) [104] was derived from the electron self-exchange rate between HRP compound II and ferric HRP (4.9 x 10 m s ) [104], The extremely large 2 value (78.0 kcal mol ) for the metal-centered electron-exchange between HRP compound II (Fe ) and ferric HRP (Fe ) is consistent with the large... 

The figure shows a frontier orbital map of metal and ligands constructed for the [RuCl6-n(PhCN)n] family, by use of equations 1 and 2. Note that the vertical axis is related to electrode potential i.e. these data are electron-transfer couples, not orbital energies, and as such they are particularly appropriate for describing both redox and optical charge-transfer phenomena. For example, the metal-centered electron/electron... 

As with other hydroperoxides, hydroxyaLkyl hydroperoxides are decomposed by transition-metal ions in an electron-transfer process. This is tme even for those hydroxyaLkyl hydroperoxides that only exist in equiUbrium. For example, those hydroperoxides from cycHc ketones (R, R = alkylene) form an oxygen-centered radical initially which then undergoes ring-opening -scission forming an intermediate carboxyalkyl radical (124) ... 

This difference is a measure of the free-energy driving force for the development reaction. If the development mechanism is treated as an electrode reaction such that the developing silver center functions as an electrode, then the electron-transfer step is first order in the concentration of D and first order in the surface area of the developing silver center (280) (Fig. 13). Phenomenologically, the rate of formation of metallic silver is given in equation 17,... 

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

Transition metal salts trap carbon-centered radicals by electron transfer or by ligand transfer. These reagents often show high specificity for reaction with specific radicals and the rates of trapping may be correlated with the nucleophilicity of the radical (Table 5.6). For example, PS radicals are much more reactive towards ferric chloride than acrylic propagating species."07... 

However, metal ions in higher oxidation states are generally smaller than the same metal ion in lower oxidation states. In the above example, the Co(ii)-N bonds are longer than Co(iii)-N bonds. Consider what happens as the two reactants come together in their ground states and an outer-sphere electron transfer occurs. We expect the rate of electron transfer from one center to another to be very much faster than the rate of any nuclear motion. In other words, electron transfer is very much faster than any molecular vibrations, and the nuclei are essentially static during the electron transfer process (Fig. 9-6). 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

It appears that Cluster C catalyzes the chemistry of CO oxidation and transfers electrons to Cluster B, which donates electrons to external acceptors such as ferredoxin. Since a crystal structure of this protein does not exist, the proposed structure of Cluster C is based on spectroscopic measurements. In some cases, the EPR spectrum of a metal center is diagnostic of the type of center. However, the EPR spectra of Cluster C are unusual. The paramagnetic states of Cluster C (Credi and Cred2) have g-values that are atypical of standard [4Fe-4S] clusters (Table III) and are similar to those in a variety of structurally unrelated systems including a t-oxo bridged ion dimer), a [Fe4S4] ... 

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