Oxidation-reduction stability relationships

Relative oxidation-reduction stability relationships, in which both the lanthanides and actinides are compared (stable high oxidation state on the left, stable low oxidation state on the right) in various media, are shown in figs. 9 and 10. 

The actinide elements exhibit uniformity in ionic types. Corresponding ionic types are similar in chemical behavior, although the oxidation-reduction relationships and therefore the relative stabilities differ from element to element. 

The donor-acceptor relationships include oxidation-reduction, complexforming, nucleophilic, electrophilic, acidobasic and other processes. A general principle of this theory, from the viewpoint of reactivity and stability of products, is that behave differently in waters. The dependence on pH of the concentrations of the various species is solved by calculation or graphically. 

The importance of the dihydro and tetrahydro oxidation states of pterins in biology has stimulated interest in the study of the chemical properties of these compounds, especially with respect to electron-transfer and radical reactions. It has become apparent, perhaps unsurprisingly, that the stability and reactivity of these oxidation states are very sensitive to substituent effects and the much greater stability of the fully conjugated pteridines is most evident. The oxidation of tetrahydropterins and the reduction of dihydropterins have become especially important in the chemistry of nitric oxide production in nature and in oxidative stress but the accumulation of relevant facts has not led so far to a detailed understanding of the chemical property relationships. Relevant information is summarized in the following section. 

Advantage has been taken of the ready accessibility of eleven para-substituted trityl and 9-phenylxanthyl cations, radicals, and carbanions in a study of the quantitative relationship between their stabilities under similar conditions.2 Hammett-type correlations have also been demonstrated for each series. Heats and free energies of deprotonation and the first and second oxidation potentials of the resulting carbanions were compared. The first and second reduction potentials and the p/CR values of the cations in aqueous sulfuric acid were compared, as were calorimetric heats of hydride transfer from cyanoborohydride ion. For radicals, consistent results were obtained for bond dissociation energies derived, alternatively, from the carbocation and its reduction potential or from the carbanion and its oxidation potential. 

The substitutionally labile complex may be generated not only by reduction but by oxidation as well. An immediate relationship of such a reaction to the ac electrolysis proceeding without generation of excited states can be recognized. The initial production of the substitutionally labile oxidation state of ML can be achieved electrochemically (67-76), chemically (75-77) or photochemically (78). In the electrochemical experiments reduction or oxidation was accomplished by a direct current. In most cases these processes are catalytic chain reactions with Faradaic efficiencies much larger than unity. Electrochemical substitution of M(CO), with M = Cr, Mo, W was carried out by cathodic reduction to M(CO) which dissociates immediately to yield M(CO). Upon anodic reoxidation at the other electrode coordinatively unsaturated M(CO), is formed and stabilized by addition of a ligand L to give M(CO)5L (68). 

The rel-HFS and rel-HF computer programs allow calculations of electronic energy levels, ionization potentials, and radii of atoms and ions from hydrogen into the superheavy region. In order to arrive at the oxidation states most hkely to be exhibited by each superheavy element and also the relative stabilities of these various oxidation states, we need to be able to relate these properties to calculable electronic properties. The relationship between reduction potentials and the Born-Haber cycle has offered an effective approach to this problem (69, 70). 

A few elements—C, N, O, S, Fe, Mn—are predominant participants in aquatic redox processes. Tables 8.6a and 8.6b present equilibrium constants for several couples pertinent to consideration of redox relationships in natural waters and their sediments. Data are taken principally from the second edition of Stability Constants of Metal-lon Complexes and Standard Potentials in Aqueous Solution (Bard et al., 1985). A subsidiary symbol pe (W) is convenient for considering redox situations in natural waters. pe°(W) is analogous to pe except that H" and OH in the redox equilibrium equations are assigned their activities in neutral water. Values for pe°(W) for 25 °C thus apply to unit activities of oxidant and reductant at pH = 7.00. pe°(W) is defined by... 

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