Kinetics and mechanism of the reduction of enneamolybdomanganate(IV) by sulfite

The kinetics and mechanism of reduction of enneamolybdomanganate(IV) by sulfite in HOAc—NaOAc buffer solution was studied by spectrophotometry. The reaction, with respect to enneamolybdomanganate(IV), is pseudo-first-order and, with respect to sulfite, is first-order. [H⁺]/k _obs increases with the concentration of H⁺ and the reaction rate increases with temperature. The rate constants and activation parameters of the rate-determining steps were evaluated. A mechanism related to the reaction is proposed.     

Kinetics and mechanism of reduction of oxo-chromium(V) salen by vanadium(IV)

The reduction of oxo-chromium(V) salen with a 40–160-fold excess of oxovanadium(IV) ([H⁺] = 0.02–0.1 M) at 25 °C has been investigated. The observed absorbance changes fitted a pseudo-first-order process. The nature of the intermediate, final product and reaction mechanism have been proposed on the basis of reaction conditions and observed rate constants. E.s.r. data support 1:1 stoichiometry with VO²⁺ in a deficiency. With an excess of VO²⁺ a Cr^III product corresponding to a two electron reduction process has been obtained. The spectral and ion exchange properties of the chromium product correspond to that of the N,N-ethylene-bis(salicylideneimine) derivative of Cr^III. The rate of formation of the final product increases with decreasing [H⁺]. The observed kinetic behavior is consistent with a mechanism involving the formation of a Cr^IV—V^V intermediate in an equilibrium step prior to the electron transfer step. The equilibrium constant for the formation of the intermediate has been estimated to be 11.2 ± 0.8 M^–1. The second-order-rate constants for the reduction of Cr^V species have been estimated to be 0.14 × 10², 0.10 × 10² and 0.05 × 10² M^–1 S^–1 at [H⁺] = 0.02, 0.05 and 0.1 M respectively. Like the Fe^II—Cr^V redox couple, the V^IV—Cr^V redox reaction also follows an inner-sphere process.     

Kinetics and mechanism of the inner-sphere reduction of hexachloroplatinate(IV) by dithionite in acetate buffer

The kinetics of reduction of hexachloroplatinate(IV) by dithionite have been examined spectrophotometrically in sodium acetate?Cacetic acid buffer medium in the temperature range 288?C303?K. The reaction is first order in both platinum(IV) species and dithionite. H⁺ ion has an inhibiting effect on the rate in the pH range 3.68?C4.80. The pseudo-first order rate constant increased upon increasing both ionic strength and dielectric constant. The suggested mechanism involves an initial transition state between two like charged ions, which then decomposes to give SO₃ ^2? through the intermediate formation of free radicals. The presence of free radicals was confirmed by performing the reaction in the presence of acrylamide. PtCl₆ ^2? is finally reduced to PtCl₄ ^2?, as confirmed by thermogravimetric analysis and IR spectrophotometry. The values of ?H^?? and ?S^?? associated with the rate-determining step have been calculated as 33?±?4?kJ?mol^?1 and ?141?±?7?JK?mol^?1, respectively. The values of ?H° and ?S° for the dissociation of HS₂O₄ ^? are 16?±?4?kJ?mol^?1 and ?14?±?7?JK?mol^?1, respectively.     

Kinetics and mechanism of the photoinitiated autoxidation of sulfur(IV) in the presence of iodide ion

The kinetics and mechanism of the photoinitiated and iodide ion-catalyzed aqueous autoxidation of sulfur(IV) has been studied in a diode-array spectrophotometer using the same light beam for excitation and detection. Light absorption of both the iodide ion and sulfur(IV) contribute to the initiation of a highly efficient radical chain reaction, the overall rate of which depends on the reactant and catalyst concentrations, the pH, and the light intensity in a complex manner. To interpret all the experimental findings, an elaborate scheme is proposed, in which the chain carriers are SO3-*, SO4-*, SO5-*, I*, and I2-*. There are three termination steps, each of them is second-order with respect to the chain carriers. Model calculations and nonlinear fitting have been used to show that the proposed scheme gives an excellent quantitative interpretation of the experimental results.     

Kinetics and mechanism of the oxidation of selenium(IV) by permanganate

Summary The oxidation of selenium(IV) by permanganate has been studied kinetically in acid, neutral and alkaline media. The reaction exhibits unit-order dependence on selenium(IV) and permanganate in all the three media. Manganese(VI) retards the reaction in alkaline medium. The rate-limiting step is the same in all the three media, but the stoichiometry is different, being 2:1 in alkaline medium, 2:3 in neutral medium and 2:5 in acid medium. Evidence has been obtained for a one electron-transfer in the rate-determining step.     

Kinetics and mechanism of the oxidation of thiosulphate by hexachloroplatinate(IV)

Summary Rate constants for the oxidation of thiosulphate by hexachloroplatinate(IV) have been measured. The kinetics of the oxidation of thiosulphate follow a second-order rate law, first order with respect to thiosulphate and first order with respect to platinum(IV). The influence of pH is small. The rates are found to depend on the nature and concentration of the cations and follow the order: Cs⁺>Rb⁺>K⁺>Na⁺>Li⁺. The activation parameters calculated from the temperature studies are: H=42.9 k J mol^–1 and S=–102 JK^–1 mol^–1. A mechanism of the reaction in terms of intermediate formation of free radicals followed by the formation of tetrathionate is postulated to explain the kinetic behaviour.     

Kinetics and mechanism of vanadium(IV) oxidation by tetrabutylammonium tribromide

The reaction between tetrabutylammonium tribromide(TBATB) and vanadium(IV) has been studied in 50% (v/v) acetic acid under second order conditions. The overall order of reaction is found to be two, unity in each reactant. The reaction involves two single-electron transfer steps generating bromine free radical in the first rate determining step. The test for the formation of free radicals in presence of added acrylonitrile was negative while added toluene increases the rate of the reaction considerably due to its conversion into benzyl bromide. The reaction is retarded by hydrogen ions as a result of protonation prior equilibria of the active reductant, vanadyl acetate. The oxidation of the vanadylsalen complex by TBATB proceeds more rapidly than that of vanadyl acetate but follows the similar kinetic behaviour. Considerable decrease in the entropy of activation of the reaction indicates formation of an ordered transition state between the two reactants and since the kinetic behaviour remains unaltered, even after the change in the ligand attached to the reductant, indicates an interaction between the reactants through the oxygen atom on the vanadyl ion.     

Kinetics and mechanism of osmium(VIII) catalyzed oxidation of tellurium(IV) by cerium(IV)

The reaction is first order in substrate and catalyst and zero order in cerium(IV). The rate decreases with increasing [H⁺] as well as with increasing ionic strength. H and S have been found to be 44.8 kJ mol^–1 and 161.8 JK^–1 mol^–1 respectively. A mechanism is proposed.

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(IV). [H⁺], . , H S 44,8 ·^–1 161,8 ·^–1–1, . .

     

Kinetics and mechanism of ruthenium(III) catalyzed oxidation of tellurium(IV) by cerium(IV)

The reaction is zero order in cerium(IV), fractional order in tellurium(IV) and first oder in ruthenium(III). While the ionic strength has no effect, the rate increases with increasing [H⁺], and decreases with increasing [HSO ₄ ^– ]. H and S are 54.4 kJ mol^–1 and –60.3 JK^–1 mol^–1, respectively. A suitable mechanism is proposed.

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(IV), (IV) (III). , [H⁺] [HSO ₄ ^– ]. H S 54,4 ^–1 –60,3 ·^–1·^–1, . .

     

Kinetics and mechanism of ruthenium(III)-catalysed oxidation of tellurium(IV) by alkaline diperiodatonickelate(IV)

The kinetics of Ru^III-catalysed oxidation of tellurium(IV) by alkaline diperiodatonickelate(IV) were studied spectrophotometrically using a rapid kinetic accessory. The reaction is a two stage process. In both the stages, the reaction is first-order with respect to [oxidant] and to [catalyst] with an apparent less than unit order, each in [substrate] and [alkali]. Periodate has a retarding effect on the reaction rate. A mechanism involving monoperiodatonickelate(IV) (MPN) as the reactive oxidant species is proposed. The data suggest that oxidation proceeds via formation of a complex between the active species of Ru^III and Te^IV, which then reacts with 1 mol of MPN in a slow step to yield the products. The reaction constants involved in the mechanism were evaluated. There is good agreement between the observed and calculated rate constants under varying experimental conditions for both the stages of reaction. The activation parameters for the slow step were calculated and discussed.     

Kinetics and mechanism of oxidation of S-phenylthioacetic acids by Ce(IV)

The kinetics of oxidation of several S-phenylthioacetic acids by ceric ammonium nitrate (CAN) in presence of perchloric acid has been studied spectrophoto- metrically in 50 %(v/v) aqueous acetic acid. The order with respect to Ce(IV) is one and the order with respect to S-phenylthioacetic acid is found to be 0.8. A linear plot of k_obs⁻¹ vs [substrate]⁻¹ with an intercept on the rate of axis suggests the formation of an equilibrium complex between the reactants prior to the rate determining step. The added acrylonitrile retards the reaction rate considerably suggesting that the oxidation process may involve a free radical mechanism. Electron-releasing substituents generally accelerate the rate, while electron-withdrawing groups retard the rate. A good correlation is found to exist between log k_1.8 and Hammett σ constants.     

Kinetics and mechanism of the mercury(II)-assisted hydrolysis of methyl iodide

The kinetics and mechanism of the reaction of aqueous Hg(II) with methyl iodide have been investigated. The overall reaction is best described as Hg(II)-assisted hydrolysis, resulting in quantitative formation of methanol and, in the presence of excess methyl iodide, ultimately, HgI2 via the intermediate HgI+. The kinetics are biexponential when methyl iodide is in excess. At 25 degrees C, the acceleration provided by Hg2+ is 7.5 times greater than that caused by HgI+, while assistance of hydrolysis was not observed for HgI2. Thus, the reactions are not catalytic in Hg(II). The kinetics are consistent with an SN2-M+ mechanism involving electrophilic attack at iodide. As expected, methylation of mercury is not a reaction pathway; traces of methylmercury(II) are artifacts of the extraction/preconcentration procedure used for methylmercury analysis.     

Carbonyl allylations by allylic chlorides utilizing a reduction of tin(IV) iodide to triiodostannate(II) species with iodide sources

Carbonyl allylations by allylic chlorides either with tin(IV) iodide and tetrabutylammonium iodide (TBAI) in dichloromethane or with tin(IV) iodide and sodium iodide in 1,3-dimethylimidazolidin-2-one at room temperature produced the corresponding homoallylic alcohols. The carbonyl allylations probably proceeded via the reduction of tin(IV) iodide to triiodostannate(II) species with iodide sources such as TBAI and NaI, which led to the construction of a tin(IV)-catalytic cycle based on regeneration of tin(IV) iodide via the transmetalation of homoallyloxytriiodotin to homoallyloxytrimethylsilane with iodotrimethylsilane.     

Kinetics and mechanism of the oxidation of guanosine derivatives by Pt(IV) complexes

The kinetics of redox reactions of the PtIV complexes trans-Pt(d,l)(1,2-(NH2)2C6H10)Cl4 ([PtIVCl4(dach)]) and Pt(NH2CH2CH2NH2)Cl4 ([PtIVCl4(en)]) with 5'- and 3'-dGMP (G) have been studied. These redox reactions involve substitution followed by an inner-sphere electron transfer. The substitution is catalyzed by PtII and follows the classic Basolo-Pearson PtII-catalyzed PtIV-substitution mechanism. We found that the substitutution rates depend on the steric hindrance of PtII, G, and PtIV with the least sterically hindered PtII complex catalyzing at the highest rate. 3'-dGMP undergoes substitution faster than 5'-dGMP, and [PtIVCl4(en)] substitutes faster than [PtIVCl4(dach)]. The enthalpies of activation of the substitution, DeltaH double dagger s, of 3'-dGMP is only 70% greater than that of 5'-dGMP (50.4 vs 30.7 kJ mol(-1)), but the entropy of activation of the substitution, DeltaS double dagger s, of 3'-dGMP is much greater than that of 5'-dGMP (-59.4 vs -129.5 J K(-1) mol(-1)), indicating that steric hindrance plays a major role in the substitution. The enthalpy of activation of electron transfer, DeltaH double dagger e, of 3'-dGMP is smaller than that of 5'-dGMP (88.8 vs 137.8 kJ mol(-1)). The entropy of activation of electron transfer, DeltaS double dagger e, of 3'-dGMP is negative, but that of 5'-dGMP is positive (-27.8 vs +128.8 J K-1 mol-1). The results indicate that 5'-hydroxo has less rotational barrier than 5'-phosphate, but it is geometrically unfavorable for internal electron transfer. The electron-transfer rate also depends on the reduction potential of PtIV. Because of its higher reduction potential, [PtIVCl4(dach)] has a faster electron transfer than [PtIVCl4(en)].     

Kinetics of the reduction of plutonium(IV) by hydroxyurea, a novel salt-free agent

The kinetics of the reduction of plutonium(IV) by hydroxyurea (HU), a novel salt free reductant, in nitric acid solutions has been studied. The observed reaction rate can be expressed as: -d[Pu(IV)]/dt=k ₀[Pu(IV)]²[HU]/[H⁺]^0.9, where k ₀ = 5853±363 (l^1.1.mol^-1.1.s^-1) at t = 13 °C. The activation energy is about 81.2 kJ/mol. The study also shows that uranium(VI) has no appreciable influence on the reaction rate. Compared with other organic reductants our experiments indicate that HU is a very fast reductant for plutonium(IV). This revised version was published online in July 2006 with corrections to the Cover Date.     

Heteropoly compounds as octahedral high valence complex. Spectrochemical properties of Waugh-structure enneamolybdonickelate(IV) and enneamolybdomanganate(IV) ions

Electronic absorption and magnetic circular dichroism (MCD) spectra in UV-vis region of Waugh-structure [XMo₉O₃₂]^6?(X = Ni(IV), Mn(IV)) ion in aqueous solution and solid IR spectra have been measured. The Ni(IV) ion in the polyanion has a low-spin d⁶ electronic configuration and the Mn(IV) ion has a d³ configuration. Visible absorption spectrum of the nickelate(IV) polyanion is interpreted to be mainly governed by charge-transfer transitions of the “ligand”, Mo₉O₃₂, to Ni(IV) ion, rather than d-d transitions, while visible absorption of the manganate(IV) polyanion is governed, to a great extent, by d-d transitions. It is indicated by the MCD spectrum that the splitting of the first d-d absorption in the manganate(IV) polyanion is due to a contribution of the spin-forbiden transition, rather than from a trigonal splitting of the spin-allowed transition. Absorption and MCD spectra in UV region are due to charge-transfer transition within a common “ligand”, which are less influenced by the kind of heteroatom, Ni(IV) or Mn(IV). The MCD pattern by the intra-ligand charge-transfer is especially characteristic of the Waugh-structure polyanions.     

Kinetics and mechanism of oxidation of tellurium(IV) by cobalt(III) in perchloric acid

Summary The kinetics of oxidation of Te^IV by Co^III have been studied in aqueous HClO₄. A mechanism presuming [Co(OH₂)₅(OH)]²⁺ to be the reactive species has been proposed, which leads to the rate-equation shown. Rate=–d[Co^III]/dt=2kKK _h ² [Co^III] _t ² [Te^IV]/[H⁺]² K_b is the hydrolysis constant of Co^III, K is the formation constant of the complex between Co^III and Te^IV and k is the rate of decomposition of that complex. E_a and S are 95.0±2.1 kJ mol^–1 and 28.3±7.1 JK^–1 mol^–1, respectively.