Oxidation-reduction reactions within metal complexes

We believe that catalysis occurs by formation of a complex between acetaldehyde, peracetic acid, and the metal ion in the 3+ oxidation state. The metal ion could be acting as a superacid as for peracetic acid decomposition, although oxidation-reduction reactions within the complex cannot be ruled out. Here again, we have found a disturbing lack of catalytic activity of other trivalent metals (aluminum, iron, and chromium). Simple acid catalysis is not as effective as proved when using p-toluenesulfonic acid and acetyl borate. This indicates that at least more than one coordination position is needed to obtain a complex of the proper configuration. 

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. 

Oxidation-reduction reactions of transition-metal complexes involve electron transfer from one complex to another. The two molecules may be connected by a common ligand through which the electron is transferred (inner-sphere reaction), or the exchange may occur between two separate coordination spheres (outer-sphere reaction). Electron transfer rates depend on the rate of ligand substitution within the reactants, the match of the reactant orbital energies, solvation of reactants, and the nature of the ligands. These reactions have... 

The release of N2 occurs within function 3. It involves the dissociation of NO (via a dinitrosyl-adsorbed intermediate), followed by subsequent formation of N2 and scavenging of the adsorbed oxygen species left from NO dissociation. The removal of adsorbed oxygen is due to the total oxidation of an activated reductant (CxHyOz). This reaction corresponds to a supported homogeneous catalytic process involving a surface transition metal complex. The corresponding catalytic sequence of elementary steps occurs in the coordinative sphere of the metal cation. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

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... 

Current approaches to metal bioremediation are based upon the complexation, oxidation-reduction, and methylation reactions just discussed. Until recently, interest was focused on technologies that could be applied to achieve in situ immobilization of metals. However, within the last few years, the focus has begun to shift toward actual metal removal, because it is difficult to guarantee that metals will remain immobilized indefinitely. 

In order for a speciation analysis to be successful, the analytes must be stable throughout the entire analysis process. Most difficult can be keeping the sample stable up to the point it is injected into the instrument. Since the analytes can be oxidized or sometimes reduced to related species within the sample, simply handling the sample may result in changes in the relative concentrations of the analytes. Preconcentration of the samples is not advised and in many cases, the samples must be treated to preserve the original state. For example, EDTA may be added in some cases to complex the metals in a sample, preventing them from oxidation or reduction reactions. Nitric... 

Reactions of coordination complexes can be divided into (1) substitution at the metal center, (2) oxidation-reduction, and (3) reactions of the ligands that do not change the attachments to the metal center. Reactions that include more elaborate rearrangements of ligand structures are described in Chapter 14 within the context of organometallic chemistry. 

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