The selective oxidation of organic sulfides to sulfoxides by oxo(salen)chromium(V) complexes in acetonitrile is overall second-order, first-order each in the oxidant and the substrate. The rate constant, k(2), values of several para-substituted phenyl methyl sulfides correlate linearly with Hammett sigma constants and the rho values are in the range of -1.3 to -2.7 with different substituted oxo(salen)chromium(V) complexes. The reactivity of different alkyl sulfides is in accordance with Taft's steric substituent constant, E(S). A mechanism involving direct oxygen atom transfer from the oxidant to the substrate rather than electron transfer is envisaged. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reaction of various sulfides with a given oxo(salen)chromium(V) complex and vice versa. Mathematical treatment of the results shows that this redox system falls under strong reactivity-selectivity principle (RSP).

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Ruthenium(II)-polypyridyl complexes of similar size but with variable reduction potential undergo efficient photoinduced electron-transfer reactions with phenolate ions in aqueous medium. All these reactions are exergonic and are in accordance with the Marcus theory of electron transfer. At high negative ∆G°Marcus inverted region is observed in this bimolecular photoinduced charge separation reaction.

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The kinetics of picolinic acid (PA)-catalysed Crvi oxidation of organic sulphur compounds have been studied with several alkyl aryl sulphides and diphenyl sulphides in acetic acid-water mixtures. The PA-catalysed oxidation follows third-order kinetics, first order each in oxidant, sulphide, and catalyst, at constant [H'] and ionic strength. A good correlation exists between log k, and Hammett (r constants for both aryl methyl and diphenyl sulphides, and the reaction constants are negative. Steric congestion at the reaction centre, sulphur, has been revealed by studies with C,H,SR (R = Me, Et, Pr, Pri, and But). Three mechanisms are proposed t o account for the observed experimental results. In aryl methyl sulphides the rate benefit (kcatkuncat)/ kuncat is the least for p-methoxyphenyl methyl sulphide and the largest for p-nitrophenyl methyl sulphide and this is in accordance with the reactivity-selectivity principle.Though, in the chromium(v1) oxidation of organic compounds, reagents such as 1,lO-phenanthroline, 2,2'-bipyridyl, and ethylenediaminetetra-acetic acid (EDTA) act as efficient catalysts,'V2 Rocek and Peng3 were the first to report that picolinic acid (PA) is a specific catalyst for the CrV1 oxidation of alcohols. In the PA-catalysed CrV1 oxidation of propan-2-01 the rate acceleration has been attributed to fast attack of PA-complexed CrV' on the alcohol. Our interest in the oxidation of organic sulphur compounds by several oxidants,"* and in particular our recent studies on the uncatalysed CrV' oxidation of organic sulphides,' prompted us to undertake a detailed investigation on the kinetics of PA-catalysed Cr"' oxidation of a number of alkyl aryl sulphides and a few diphenyl sulphides in acetic acidwater mixtures. Incidentally this study appears to be the first report on the effect of substituents in PA-catalysed Crvl oxidations.

The rate constants for the photoinduced electron transfer reactions of Ru(II)-polypyridyl complexes (k q ) with 2,6-disubstituted phenolate ions in aqueous acetonitrile are highly sensitive to change in the (Ru(NN) 3 2`) bulkiness of the ligand in as well as the phenolate ion. The decrease in value with the increase in Ru(NN) 3 2`k q the size of the ligand in and the phenolate ion is ascribed to the decrease in the electronic coupling Ru(NN) 3 2m atrix element, between the donor and acceptor with the increase in the electron transfer distance. The o H DA o, hydrophobic interaction or possible pÈp stacking between the pyridine rings of and the aryl Ru(NN) 3 2m oiety of ArO~leads to less steric e †ect.

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