Oxidation state, changes under reaction

An example where changes in pretreatment steps are very drastic is in hydrotreatment. The most applied catalysts are based on a combination of molybdenum and cobalt sulphides. They are manufactured in the oxidic state, and under reaction conditions these oxides are not stable and are transformed into the sulphidic state. In practice, the oxidic catalyst is presulphided, either in the reactor (by adding H2S or a compound that is easily converted into H2S), or ex situ by the manufacturer. These presulphiding procedures often involve temperature-programmed reactions. 

The mechanism will vary in precise detail according to the metal. In the case of ruthenium complexes, it is quite common to observe a conproportionation and the formation of a ruthenium(iv) intermediate. In other cases, the unavailability of the metal oxidation states precludes reaction. For example, cobalt(m) complexes of cyclam cannot be oxidised to imine species because although a cobalt(ii)/cobalt(m) couple is possible, the cobalt(n) oxidation state is not accessible under oxidative conditions. In the case of metal ions which can undergo two oxidation state changes, alternative mechanisms which do not involve radical species have been suggested. 

Small differences in the chemistry of the lanthanides are observed due to the gradually changing radial size of the elements, which decreases from 1.061 A for La to 0.848 A for Lu (77). Differences in chemistry also occur for the four elements in the series which have nontrivalent oxidation states accessible under normal reaction conditions Ce (4/°), Eu " (4/ ), Yb " (4/ and Sm (4 f ). Since Ce is a strong oxidizing agent and the... 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

Oxidation-reduction reactions may affect the mobility of metal ions by changing the oxidation state. The environmental factors of pH and Eh (oxidation-reduction potential) strongly affect all the processes discussed above. For example, the type and number of molecular and ionic species of metals change with a change in pH (see Figures 20.5-20.7). A number of metals and nonmetals (As, Be, Cr, Cu, Fe, Ni, Se, V, Zn) are more mobile under anaerobic conditions than aerobic conditions, all other factors being equal.104 Additionally, the high salinity of deep-well injection zones increases the complexity of the equilibrium chemistry of heavy metals.106... 

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. 

As shown in Table V, a number of Fe S-containing proteins perform reactions other than redox or electron transfer. That is, the function of the cluster does not include a change in oxidation state, even as a transient step in catalysis. This role is best illustrated by aconitase, one of the most extensively studied Fe S proteins, regardless of function. The elegant recent work on this enzyme is largely under the guiding hand of H. Beinert and is summarized in the Krebs Memorial Lecture (Beinert and Kennedy, 1989). 

The E (vertical) axis is a reflection of the potential values in volts (v) of reduction half-reactions describing the conditions under which changes in the aqueous oxidation state of the element occur. These E values range from -1-3.00 V to —4.00 V. The pH (horizontal) axis gives pH values ranging from a pH of —1.0 (10 molar hydrogen ion) to a pH of 15.0 (10 molar hydrogen ion). The sloped dashed lines have to do with the behavior of the solvent water. This will be discussed in detail later. 

A consideration of these relationships reveals8 that because E° is a thermodynamic parameter and represents an energy difference between two oxidation states and in many cases the spectroscopic or other parameter refers to only one half of the couple, then some special conditions must exist in order for these relationships to work. The special conditions under which these relationships work are that (a) steric effects are either unimportant or approximately the same in both halves of the redox couple and (b) changes in E° and the spectroscopic or other parameters arise mainly through electronic effects. The existence of many examples of these relationships for series of closely related complexes is perhaps not too unexpected as it is likely that, for such a series, the solvational contribution to the enthalpy change, and the total entropy change, for the redox reaction will remain constant, thus giving rise to the above necessary conditions. 

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