Redox metal-ligand reactions

The most important types of reactions are precipitation reactions, acid-base reactions, metal-ligand complexation reactions, and redox reactions. In a precipitation reaction two or more soluble species combine to produce an insoluble product called a precipitate. The equilibrium properties of a precipitation reaction are described by a solubility product. 

These complexes combine a central metal in a high oxidation state with a redox active ligand (catechol). This combination arises from the idea that the electronic perturbation induced in the metal complex by reaction with dioxygen can discharge itself ... 

Vance and Miller et al. have shown that the inactivity of enzyme is due to changes in the redox potentials of the enzyme. In order to dismute 02 the redox potential of the enzyme must lie between the E° values for the reactions shown in Equation (3). The E° value of the E. coli MnSOD enzyme is 0.290 V and that for the FeSOD is 0.220 V. The Fe-substituted form of the Mn enzyme has F ° =—0.240 V and Mn-substituted FeSOD has ii° >0.960V. These values are outside the required range and the changes in redox potentials are not due to changes in the metal ligands. Mutations of His-30 and Tyr-34, two conserved residues in the immediate vicinity of the metal binding site, do not alter the redox potential of the enzyme either " ... 

The formation of ligated transition metal ions at unstable high states of oxidation, its implications in the mechanisms of metal-catalyzed autoxidation, and the effect of configuration of a metal-ligand system on its redox stability have been pointed out. These considerations may be helpful in interpreting more complex metal-ligand systems including metal-enzyme reactions. 

Atom transfer radical polymerization (ATRP) [52-55]. Active species are produced by a reversible redox reaction, catalyzed by a transition metal/ligand complex (Mtn-Y/Lx). This catalyst is oxidized via the halogen atom transfer from the dormant species (Pn-X) to form an active species (Pn ) and the complex at a higher oxidation state (X-Mtn+1-Y/Lx). 

For efficient regeneration, the catalyst should form only labile intermediates with the substrate. This concept can be realized using transition metal complexes because metal-ligand bonds are generally weaker than covalent bonds. The transition metals often exist in different oxidation states with only moderate differences in their oxidation potentials, thus offering the possibility of switching reversibly between the different oxidation states by redox reactions. 

Any surface reaction that involves chemical species in aqueous solution must also involve a precursory step in which these species move toward a reactive site in the interfacial region. For example, the aqueous metal, ligand, proton, or hydroxide species that appear in the overall adsorption-desorption reaction in Eq. 4.3 cannot react with the surface moiety, SR, until they leave the bulk aqueous solution phase to come into contact with SR. The same can be said for the aqueous selenite and proton species in the surface redox reaction in Eq. 4.50, as another example. The kinetics of surface reactions such as these cannot be described wholly in terms of chemically based rate laws, like those in Eq. 4.17 or 4.52, unless the transport steps that precede them are innocuous by virtue of their rapidity. If, on the contrary, the time scale for the transport step is either comparable to or much longer than that for chemical reaction, the kinetics of adsorption will reflect transport control, not reaction control (cf. Section 3.1). Rate laws must then be formulated whose parameters represent physical, not chemical, processes. 

Historically, the development of ET theory has been based on inorganic systems, in which the (metal-ion) redox centers are surrounded by coordinated ligands [52]. In those cases in which no new metal-ligand bonds are formed or bond breakage is observed, the interaction between the redox centers is weak (usually Hda < 200 cm-1), and such reactions are conventionally designated as outer-sphere (OS) electron transfer [52, 53]. 

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