Ion-Ligand Redox Reactions

The oxidations of a series of catechol derivatives (HaCat) by iron(iii) in aqueous perchloric acid media, 

Studies of the redox equilibria between iron(m) and 1,2-dihydroxynaphthalene-4-sulphonate have been reported. At pH 2, iron(m) spontaneously oxidizes the catechol, but in more basic solution complexes of iron(ui) with one and two equivalents of substrate were detected. The absence of 1 3 species below pH 7 suggests that the complexes might bind molecular oxygen and that the system is well suited to a catalytic role in the aerial oxidation of organic compounds. 

Under anaerobic conditions, iron(iii) oxidizes sulphide co-ordinated to chro-mium(m) by two parallel pathways  

The contribution from each reaction varies with acidity but the major product is [CrS2HFe] + in which the metal-ion oxidation states are ambiguous. A dependence on IH+] in the experimental rate law 

The kinetics of reduction of horse-heart ferricytochrome c [Cyt by catechol follow the rate expression 

Quantitative data for metal ion-ligand redox reactions are collected in Table 1 on p. 50. 

The mechanism of formation of the intermediate complex in the oxidation of hydrogen thiosulphate by Cr i has been described, the equilibrium constant (1.1 X 10 1 mol ) for the reaction 

The reaction of a mixture of oxalic acid and isopropyl alcohol with chromium(vi) occurs at a rate much greater than that for either of the two substrates alone,both substrates undergoing oxidation. In the presence of free-radical scavengers, the products are a 1 1 ratio of acetone and CO2, indicating a two-electron oxidation of the alcohol and a one-electron transfer for the oxalic acid. In the absence of acrylonitrile, however, the yields of these products are not compatible with a simple mechanism. The suggested mechanism is considered to involve a one-step, three-electron oxidation with the change in the chromium reactant from CrVi- -Crin over a wide concentration range, the rate may be described by the rate law 

The oxidation of ethylene glycol has been shown by e.s.r. techniques to involve the fonnation of intermediate Cr species. Kinetic data suggest that the intermediate is formed by the reaction of Cr and Cr. Addition of water to the reaction medium affects the stability of the complex, which has a structure (1) consistent with eight equivalent protons in the e.s.r. spectrum. 

Maximum concentrations of the species were obtained when the reaction was carried out at low pressure with removal of the water formed in the course of the reaction. In this way the Cr species stabilized by ethylene glycol was stable at room temperature for several days. When the reaction was studied in dioxan, a variety of hydroxy-carboxylic acids were shown to give rise to Cr signals whereas oxalic acid and mercapto-carboxylic acids do not. The kinetics of the oxidation of a-hydroxybutyric and other a-hydroxy-acids in water have been investigated,with no evidence for intermediate complex formation. Failure to detect any reduction of HgCl2 has been attributed to the absence of free radicals, the authors considering the rate-determining process to involve the formation of a carbonium ion which yields the ketone product by release of a proton. 

The initial nitrogen oxidation product is NgHj, which is thought to disproportionate. A bimolecular reaction takes place involving the Cr formed in the two-electron redox step. Some Li+ catalysis similar to that of H+ occurs. By investigating the 

Two studies have been made of the chromium(vi) oxidation of thiocyanate, - in which the reported dependence of the ligand concentration is substantially different. In keeping with the earlier observation that a complex between Cr and SCN is sufficiently stable with respect to redox at low [H+] to allow temperature-jump studies to be made (and hence to yield a value for the formation constant), Muirhead and Haight found an immediate increase in absorbance on mixing the reactants in the stopped-flow apparatus. The spectrum of the intermediate was derived using the continuous-flow method. Assuming the oxidation reaction to proceed via this complex, the rate law may be written in the form 

In the oxidation of arsenic(iii) in perchlorate media the reaction order of less than unity with respect to reductant is interpreted in terms of the formation of an unstable 1 1 intermediate. Activation parameters are essentially constant (Ea = 9.3 0.5 kcal mol ) at both high (excess) and low [Cr ] and the rate is increased on addition of sodium hydrogen phosphate. This latter observation may reflect an acid-catalytic route described earlier. The mechanism at low [Cr T with reductant in excess is considered to involve the rate-determining decomposition of the 1 1 complex to give Cr and an arsonium species of the type 

In the presence of oxidizable metal ions, however, e.g. Ce or Mn ) there is always the possibility of trapping the intermediates in competition with the ligands reacting to reduce the Cr. Chromium(v) plays an important role in oxidations of this type and a long-lived Cr intermediate has been identified in the reaction with oxalic acid. Using both e.s.r. and difference absorbance spectra, it has been shown that there is a significant build-up of a complex in moderately acidic media. The data 

The invariance of the rate on aldehyde concentration is consistent with only Cr v acting as an oxidant for this substrate. Evidence has been presented for the reaction of this reagent v/ith isopropyl alcohoE as an essential step in the Cr oxidation of this substrate in acetic acid. A stable Cr alkoxide of a secondary alcohol has been prepared. The complex tetrakis-(3,3-dimethyl-2-butoxy)chromium(iv) is, however, sensitive to moisture and oxygen. The difference in its stability with respect to primary and secondary alcohols results from the presence of the bulky t-butyl groups which probably prevent the rearrangement of the molecule to the correct conformation required for hydrogen transfer in the oxidation step. 

The format used in this chapter is similar to that in previous volumes. An attempt has been made to cover the area of reactions of metal ions in high oxidation states with inorganic and organic substrates as comprehensively as possible although some selectivity has been imposed. Where possible compilations of data have been assembled to allow for comparison of rate data. 

In the period covered by this volume, a book chapter and a whole book dealing with aspects of metal ion-ligand redox reactions have been published. 

The kinetics of the oxidation of hydrazine by jHCrOJ have been reported. Only molecular nitrogen is formed in the overall reaction, indicating a simultaneous two-electron-transfer process  

The rate is first-order in both oxidant and substrate and the proposed mechanism involves rate-determining formation of a weak complex  

In a second paper on the same reaction, kinetic evidence for complex formation was obtained. The rate law in sulphate media, 

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Metal Ion-Ligand Redox Reactions discussed.The reaction ... 

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