Oxidation state charge models

Consider the closely related ion [FeCHiO/e] ". The only difference is in the formal oxidation state of the metal ion. If an ionic model is assumed (9.6), the charge on the metal center is +2. A purely covalent model results in the placing of a formal quadruple negative charge upon the iron center (9.7). To satisfy the electroneutrality principle, and establish a near-zero charge on the metal, each oxygen atom is... 

The so-called formal oxidation number is the widest known operational concept for oxidation state. This number can be determined from the stoichiometry of the compound by some simple mles (the oxidation number of O and H being — 2 and + 1, respectively the sum of the oxidation numbers is equal to the total charge of the species under consideration). The statement that the formal oxidation number of Cu in CUCI2 is + 2 was theoretically significant in a simple but obsolete bonding model in which CuC was thought to be composed of isolated Cu and Cl" ions. 

We close this section with a note on the influence of pH on reduction potentials. Many redox reactions are pH-dependent, which can be understood with reference to the simple model in Figure 13.4, in which a redox compound in its oxidized state has a pK,t for proton dissociation that is different from (i.e., lower than) the corresponding value for its reduced state the positive charge of Xox is higher than that of Xred, so it is more difficult for Xox to accept a proton (i.e., its pKa is lower). The °(pH) is now... 

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. 

ISP contain four basic core structures which have been characterized crystallographically both in model compounds and in ISP (Rao and Holm, 2004). These are (Figure 13.15), respectively, (A) rubedoxins found only in bacteria, in which the [Fe-S] cluster consists of a single Fe atom bound to four Cys residues—the iron atom can be in the +2 or +3 valence (B) rhombic two-iron-two-sulfide [Fe2-S2] clusters—typical stable cluster oxidation states are +1 and + 2 (the charges of the coordinating cysteinate residues are not considered) ... 

Fig. 10 Formation of the bipolaron (= diion) state in PPy or PTh upon oxidation. In the model, it is assumed that the ionized states are stabilized by a local geometrical distortion from a benzoid-like to a quinoid-like structure. Hereby, one bipolaron should thermodynamically become more stable than two polarons despite the Coulombic repulsion between two similar charges.

The ionization state of the coenzyme is also important. During reduction a charged pyridinium species is created while during oxidation the charge is lost. Thus, more polar environments favor reduction while more hydrophobic conditions favor oxidation [69]. Therefore the apoenzyme environment and model system scaffolds must not only enhance the reactivity of the coenzyme, but must also address these issues of equilibrium and stability. 

---