The kinetics of oxidation of thiosulfate to tetrathionate by trans-dihydroxotetraoxoosmate(VII1) in aqueous alkaline media have been studied. The oxidation follows a rate expressionwhere K a is the formation constant of trans-dihydroxotetraoxoosmate (VIII), and K2 and ka, respectively, represent the formation constants of the intermediate complex involving Os(VII1) and S20;and its decomposition constant. The K h , K P , and k3 values have been computed to be (19.5 f 3) dm3/mol, (6.12 f 0.5) and (3.32 f 0.3) X lo-' dm3/mol s a t 303 K, and I = 0.32 mol/dm3, respectively. The rate law is consistent with a mechanism envisaging the equilibrium formation of an intermediate complex involving Os(VII1) and S*O$-, followed by a rate-determining decomposition of the complex with concomitant electron transfer. 1002PANDA, NEOGI, AND RAMASWAMY complexes [2-61 formed prior to electron transfer or without a change in the inner coordination sphere of the metal [7-111. Keeping this in view, the electron transfer in the OS( VIII)-S20;-system has been studied by stopped-flow spectrophotometry. Materials and MethodsAll the chemicals used, except Os04 (which was a sample of Johnson and Matthey), were of AnalaR grade. Doubly distilled water was exclusively used for the preparation of solutions. Stock Os(VII1) solution, prepared by dissolving Os04 in a weakly aqueous alkaline solution, was estimated by iodometry [12] and stored in a cool dark place. The solutions for kinetic runs were prepared from this stock solution by dilution. The ionic strength

The kinetics of electron transfer from phenylhydrazine(S) to tris-(dimethylglyoximato) nickelate(IV), Ni(dmg)i-(dmg2-= dimethylglyoximate dianion), have been studied in aqueous medium in the range of 6.21 I pH I 12.2. The kinetics exhibit a pseudo-first-order disappearance of Ni(dmg)g-when excess S is present. The pseudo-first-order rate constants hobs are almost linearly dependent on [S]o for varying concentrations of the reductant. At constant [S]O, the k,b,-pH profile is U shaped. The kob values register a decrease as the [H+] is increased in the pH range of -12.2-9.5, remain almost constant (minimum) in the range of -9-8, and then again linearly increase as [H+] is increased in the pH range of -7-6.21. Results are interpreted in terms of a probable mechanism involving outer-sphere electron transfer from the phenylhydrazine and phenylhydrazinium cation species to the unprotonated and one-protonated species of the Ni(IV) complex. The reduction rate appears to be dependent on the nature of the species (unprotonated and one-protonated) of the oxidant Ni(IV) complex. The phenylhydrazinium cation reduces the Ni(IV) complex at least one order of magnitude faster than does the neutral reductant species. The major product of the oxidation of phenylhydrazine by the Ni(IV) complex is 4-hydroxyazobenzene with a small amount of phenol.

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Potassium tris(dimethylglyoximato)nickelate(iv), K2[Ni(dmg),], has been prepared in aqueous alkaline medium from nickel(i1) and dimethylglyoximate (dmg2-) in the presence of an excess of two-electron oxidant. Electronic, i.r., and 1 H n.m.r. spectra are in conformity with an octahedral (D3) geometry for this compound, with dmg2-units bonded through nitrogen donors. The lack of any significant e m . signal establishes that the central nickel is in the oxidation state IV. The formation of [Ni(dmg),12-, from Nil1 and dmg2-in the presence of hypoiodite generated in situ from tri-iodide and hydroxide, exhibits pseudo-first-order kinetics. The rate constants of formation, kobs., show linear dependence on [oxidantlo in the concentration range employed. At [OH -J = 4.8 x 1 0-2 rnol dm-3, the kobs. values show a linear increase with increasing [dmg2-lo, tending to limiting values at higher relative [dmg2-lo.The kobs. values remain almost constant with decreasing [OH-] in the range 0.105 2 [OH-] 2 1.1 x rnol dm-3, but exhibit a significant decrease with decrease in [OH -3 in the range 1.1 x 1 0-2 > [OH -1 2 6 x 1 0-4 rnol drn-,. The kinetic results are interpreted in mechanistic terms.Dioximes are good ligands for stabilization of higher oxidation states of transition metals, e.g. nickel(~v),'-~ because of their rr-donor character. Simek reported that a solution of nickel(n) dimethylglyoximate in 1-2 rnol dm-' KOH was easily oxidized by oxidants like hypoiodite, hypobromite, and peroxodisulphate, resulting in a red solution of a diamagnetic Nil" complex. Use of lead dioxide in oxidizing a mixture of nickel@) sulphate and dimethylglyoximate in 1.5 mol dm-j NaOH also yielded a red solution, which was shown from reversible polarographic studies to be a NilV complex.6 On the other hand, the product of pulse-radiolytic treatment of an N,Osaturated solution of nickel(11) sulphate and dimethylglyoximate, in 0.5 mol dm-j NaOH or 4 mol dm-j NH40H, displayed an intense band at ca. 450 nm; the reaction showed pseudofirst-order kinetics, and the product was assumed to be a Ni"' species since the oxidation was brought about by one-electron oxidants such as OH or NH2 radicals.' The reaction of [Ni-(Hdmg)2] with bromine or iodine in pyridine was reported to give a compound originally believed a to contain nickel(Iv), but later shown to be a nickel(n1) species.' On the basis of e.s.r. studies of the red solution, Marov et al.1° suggested that whereas peroxodisulphate exclusively oxidized a mixture of Ni" and dmg2-to a NilV complex, some Ni"' species was obtained in solution on oxidation o f a mixture of Nil1 and dmg2by OX-(X = C1, Br, or I) or H202 in 3 mol dm-j OH-; when the oxidizing agent was hypoiodite, the maximum accumulation of the Nil1' species (20-30%) in solution was observed with a reactant ratio (Ni" : dmg2-: 01-) of 1 : 6 : 1.8. The stoicheiometry, structure, and metal oxidation state of the red solution were the subjects of many investigations, the distribution of Ni"' and/or NilV being strongly dependent on the experimental...

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