- Reactions of vanadium(IV) and (V) with s2 metal-ion reducing centers
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The s2 centers, Sn(II), Ge(II), and In(I) reduce VO2+ rapidly and quantitatively to VO2+, and In(I) converts VO2+ (much more slowly) to V3+. Sn(II) and Ge(II) react measurably with VO2+ only in chloride media in the presence of added Cu(II). Arguments are presented that the V(v) reductions are initiated by a two-unit reduction to V(III) (via a hydroxo bridge), followed by a rapid comproportionation (VIII + VV → 2 VIV). The Cu(II)-catalyzed V(IV)-Sn(II) and V(IV)-Ge(II) reactions at high [Cl-] involve preliminary conversion of the catalyst to Cu(I), which then reduces V(IV), and kinetic profiles of the Ge(II) system point to participation of chloride-bound Ge(III) as well.
- Yang, Zhiyong,Gould, Edwin S.
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- Bimolecular homolytic reactions of alkylcobalt complexes
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The reactions of RCoL1(H2O)2+ (R = CH3, C2H5 and n-C3H7; L1 = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) and CH3CoLsu
- Lee, Shaoyung,Ku, Tsung Yao
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p. 2901 - 2905
(2008/10/09)
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- Molybdenum and copper catalysis of reductions by titanium(II) and titanium(III)
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Reductions of vanadium(iv), benzoquinone, and tri-iodide, both by titanium(iii) and by titanium(ii), are catalyzed by molybdenum(vi). The VO 2+-Ti(ii) reaction is catalyzed by copper(ii) as well. Reactions of Ti(ii) with the oxidant in excess y
- Yang, Zhiyong,Gould, Edwin S.
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p. 396 - 398
(2007/10/03)
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- Reactions of tris(oxalato)cobaltate(in) with two-electron reductants
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The tris(oxalato)cobaltate(in) complex [Co(C2O4) 3]3-, EoCoIII/II = +0.57 V) is readily reduced by the 2e- reagents, Sn(II) and Ge(ii), in contrast to (NH3)5CoCl2+ and (NH3) 5CoBr2+, which are unreactive toward these donors. Rates for the oxalato oxidant are only 10-3-10-2 as great as those for vitamin B12a (aquacob(III)alamin, Eo +0.35 V at pH 1), in accord with the suggestion that reductions of corrin-bound cobalt(III) by Sn(II) and Ge(II) occur predominantly through an additional path involving Co(I). Reductions of the oxalato complex by 2e- donors are taken to proceed by initial formation of odd-electron intermediates (e.g., Sn III and GeIII) which react rapidly with CoIII. Such a two-step sequence is in keeping with the observed behavior of the rare reductant, TiII, which is found to be oxidized by [Co(C 2O4)3]3- more slowly than (independently prepared) Ti(III) under comparable conditions.
- Yang, Zhiyong,Gould, Edwin S.
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p. 3601 - 3603
(2007/10/03)
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- 2′-Hydroxyacetophenonebenzoylhydrazone as an analytical reagent for the spectrophotometric determination of vanadium(III)
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2′-Hydroxyacetophenonebenzoylhydrazone (HABH) forms a light red 1:2 (V : HABH) complex with VIII species at 50-60° in 0.12-0.20 M CH3COOH medium which is extracted into benzene and has λmax at 465 nm. The molar absorptivity, Sandell's sensitivity and standard deviation arc 1.05 × 104 dm3 mol-1 cm-1, 0.0049 μg V cm-2 and ±0.0006 respectively, at 465 nm. Beer's law is obeyed over the concentration range 0-1.5 μg V ml-1. Large number of elements do not interfere. The method can be used to determine vanadium in a wide variety of synthetic and technical samples including alloyed steel and reverberatory flue dust.
- Agnihotri,Dass,Mehta
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p. 165 - 167
(2007/10/03)
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- Kinetics of oxidation of iodide by vanadium (V)
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Kinetics of oxidation of iodide ion by VV under uncatalysed and RuIII catalysed conditions in aqueous perchloric acid medium have been studied. The reaction is first order in [VV] and first order in [I-]. With [H+], the reaction shows a complex behavior of 1.5 order till [H+] is 0.5 M and second order beyond that concentration. In the case of RuIII catalyzed oxidation, the reaction exhibits a dual character of first order and zero order in [VV]. The first order component shows 1.5 order in [I-] first order in III> and 1.5 order in [H+]. The zero order component shows first order in [I-], first order in [RuIII] and independent of [H+]. No catalysis has been observed with OsVIII. Suitable rate laws have been postulated based on the observations.
- Nadh, R. Venkata,Sundar, B. Syama,Radhakrishnamurti, P. S.
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- Kinetics and mechanisms of the redox reactions of the hydroperoxochromium(III) ion
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The reactions of the hydroperoxochromium(III) ion, (H2O)5CrO2H2+ (CrO2H2+), with Fe2+, VO2+, V2+, Cu+, Ti3+, Co([14]aneN4)2+, Co(Me6[14]aneN4)2+, Co(tim)2+, and [Ru(NH3)6]2+ have been studied in acidic aqueous solution. The reactions are accompanied by large negative entropies of activation, -110 J mol-1 K-1 for Fe2+ and -85 J mol-1 K-1 for Ti3+. All the reactions studied follow an isokinetic relationship in that ΔH? is a linear function of ΔS?. The same is true for the analogous reactions of H2O2. It is proposed that the reactions of CrO2H2+ take place by an inner-sphere, Fenton-type process yielding pentaaquaoxochromium(IV), (H2O)5CrO2+ (CrO2+), as an intermediate. The reactivity of CrO2H2+ as an oxygen transfer reagent is about 20 times greater than that of H2O2. For example, the reactions with (en)2CoSCH2CH2NH22+ to yield (en)2CoS(O)CH2CH2NH22+ have rate constants 20.5 ± 0.4 M-1 s-1 (CrO2H2+) and 1.36 M-1 s-1 (H2O2), both in 0.1 M HClO4 at 25°C. The chromyl ion, CrO2+, oxidizes CrO2H2+ to CrO22+ with a rate constant of (1.34 ± 0.06) × 103 M-1 s-1 in 0.10 M HClO4 in H2O and 266 ± 10 M-1s-1 in D2O.
- Wang, Wei-Dong,Bakac, Andreja,Espenson, James H.
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p. 5034 - 5039
(2008/10/08)
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- Copper colloids stabilized by water-soluble polymers. Part II. Their application as catalysts for dihydrogen evolution
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The kinetics of catalytic dihydrogen evolution from acidic aqueous solutions of strong one-electron reductants Vaq2+ and the cation radical of methylviologen (MV+.) in the presence of polymer-stabilized copper colloids was
- Savinova,Savinov,Parmon
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p. 231 - 248
(2008/10/08)
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- Cobalt-catalyzed evolution of molecular hydrogen
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Solutions of chromium(II), europium(II), or vanadium(II) chloride in hydrochloric acid evolve molecular hydrogen rapidly in the presence of trace concentrations of the cobalt(II) macrocycle Co(dmgBF2)2. The stoichiometry for Cr(II) corresponds to the net reaction Cr2+ + Cl- + H+ = CrCl2+ + 1/2H2. The kinetics are described quite adequately by the Michaelis-Menten scheme. Kinetic studies of the reaction were made during the pre-steady-state phase, during which an intensely absorbing intermediate forms, and also at longer times during the steady-state phase when the pseudo-steady-state concentration of the intermediate slowly declined as the substrate was consumed. Arguments are given in support of the intermediate being [(H2O)5Cr-Cl-Co(dmgFB2)2] +. Its dissociation leads, in acidic solution, to the hydridocobalt complex HCo(dmgBF2)2, which is responsible for H2 formation. Bromide ions, but not perchlorate, also give catalytic H2 production, whereas iodide forms a ternary complex that does not decompose.
- Connolly, Philip,Espenson, James H.
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p. 2684 - 2688
(2008/10/08)
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- Kinetics and mechanism of oxidation of vanadium (2+) by molecular oxygen and hydrogen peroxide
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The reaction between hexaaquovanadium(II), V(H2O)62+, and molecular oxygen has been studied by the stopped-flow method in 0.12 M perchloric acid and in solutions containing 0.1 M sulfate ion. The kinetics and stoichiometry of the reactions are consistent with a general oxidation mechanism for divalent transition-metal ions proposed by Ochiai.2 The following kinetic parameters have been determined: k2(2V2+ + H2O2) = 17.2 ± 2.0 M-1 s-1; k3(V2+ + O2) = (2.0 ± 0.2) × 103 M-1 s-1; k4((V·O2)2+ estimated dissociation) = 100 ± 50 s-1; k5((V·O2)2+ + V2+) = (3.7 ± 0.5) × 103 M-1 s-1; k-5((V·O2·V)4+ dissociation) = 20 ± 5 s-1; k6((V·O2·V)4+ decomposition) = 35 ± 5 s-1. At low V2+ concentration (2+, are produced/mol of oxygen consumed. At higher [V2+], a limiting ratio of Δ[VO2+]/Δ[O2] = 2 is approached and a limiting rate constant for VO2+ formation of 40 ± 5 s-1 is reached in both sulfate and perchlorate solutions.
- Rush, James D.,Bielski, Benon H. J.
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p. 4282 - 4285
(2008/10/08)
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- Equilibrium and kinetic studies of substitution reactions of Fe(TIM)XY2+ in aqueous solution
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Equilibrium and rate constants for substitution reactions of some bisligated complexes of Fe(TIM) (TIM is 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) in aqueous acetonitrile solutions are reported at 23°C and an ionic strength of 0.5 M. The equilibrium constant for the replacement of CH3CN by H2O (K1) in Fe(TIM)(CH3CN)22+ is 0.023 ± 0.002 whereas the same replacement in Fe(TIM)(CO)CH3CN2+ (K3) is 1.1 ± 0.1. The mechanism and the rates of establishment of these equilibria are dramatically affected by the π-accepting nature of the trans ligand, namely CH3CN or CO. Substitution trans to CH3CN is very rapid, with equilibration occurring on the time scale of milliseconds. On the other hand, the rate of approach to equilibrium in the replacement of CH3CN for H2O in Fe(TIM)(CO)CH3CN2+ is complicated by two paths, one in which the ligand trans to the CO is directly replaced and the other in which the CO leaves, equilibration of the coordination sphere of the Fe(II) center occurs, and CO then recoordinates. This rate of approach to equilibrium occurs with a rate constant dependent upon the [CH3CN], being about 3 × 10-3 s-1 at a [CH3CN] of 1.0 M. In delineation of the complexities of the rate of approach to the equilibrium described by K3, the rate constant for CO loss from Fe(TIM)(CO)H2O2+ was determined to be 3.6 × 10-3 s-1, and rate constants for all other steps involved were determined.
- Butler, Alison,Linck
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p. 2227 - 2231
(2008/10/08)
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- Reduction of α-Keto Acids by Low-Valent Metal Ions. 2. Reaction of Aqueous Vanadous Ion with Pyruvic and Phenylglyoxylic Acids
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Vanadium(II) ion reacts with pyruvic acid and phenylglyoxylic acids in aqueous acidic solutions.The first stage of both reactions corresponds to a rapid equilibrium between the V(II) ion and the organic molecule.The second stage corresponds to the reducti
- Konstantatos, J.,Vrachnou-Astra, E.,Katsaros, N.,Katakis, D.
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p. 3035 - 3040
(2007/10/02)
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- Electron transfer. 44. Decreases in the effectiveness of redox catalysts with use
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Pyridine derivatives, which, when uncoordinated, catalyze outer-sphere electron-transfer reactions between metal centers, often undergo loss of catalytic activity with use. The active intermediates in such catalyzed reactions are radicals (cat?) formed by initial reaction of the catalysts with the primary reducing centers. This study examines the deterioration of a number of such catalysts in reactions of Eu2+ and V2+ with (NH3)5Co(py)3+. This deterioration, when it occurs, arises from interaction of the catalyst and the reducing center; it does not require Co(III). Degrees of deterioration vary widely. The very powerful catalysts derived from 2,4-pyridinedicarboxylic acid (III) lose much or all of their catalytic activity during the course of a single 1-min run with Eu2+ in excess, whereas catalytic erosion of isonicotinic acid (I), its esters, and its nitrile is negligible after treatment with excess reductant for 10-20 min. Erosion is much more marked with Eu2+ than with the less strongly reducing V2+ and is much less severe when the oxidant, rather than the reductant, is taken in excess. Deterioration in Eu2+ systems may be decreased strikingly by addition of excess Eu3+ and that in V2+ systems by addition of V3+. The spectra of the products formed when the more fragile catalysts react rapidly with Eu2+ (in the absence of Co(III)) correspond to those formed by reduction of the catalysts with zinc amalgam, which is presumed to be a two-electron reductant. The more robust isonicotinate catalysts are not affected by Eu2+ under similar conditions. Evidence is presented in support of two attrition mechanisms. The catalytic deterioration of 2,4-pyridinedicarboxylic acid in Eu2+ systems appears to involve disproportionation of catalyst-radical pairs (2cat? + 2H+ → cat + catH2), converting one member of each pair to an inactive dihydro species and returning the other to the catalyst pool. Measurements of the rate of deterioration of this catalyst allow us to estimate the specific rate for the disproportionation as 5.8 × 108 M-1 s-1. In systems featuring less fragile catalysts (e.g., isonicotinamide) the steady-state concentration of the radical, cat?, is so low that attrition, if it takes place at all, occurs mainly by reductive deterioration (cat? + Eu2+ →2H+ Eu3+ + catH2). In two such instances, comparison of kinetic runs using catalyst preparations that have undergone partial attrition permits an estimate of the specific rates of such deteriorative processes. Although both modes of attrition are presumed to occur in each catalytic system, bimolecular disproportionation appears to compete most favorably with reductive deterioration when the extent of reduction of the catalyst to its radical is greatest.
- Radlowski,Chum,Hua, Louise,Heh, Jack,Gould
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p. 401 - 407
(2008/10/08)
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- The intermediate in the reaction between vanadium(II) and vanadium(IV)
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When V(II) and V(IV) are mixed in acid perchlorate solutions, a highly colored substance forms which has been shown to be an intermediate in the reaction V+2 + VO+2 + 2H+ = 2V+3 + H2O and to be VOV+4, a hydrolytic dimer of V(III). The rate of formation of VOV+4 is given by k1[V+2] [VO+2] and its rate of reaction with acid is given by k2[VOV+4] [H+]. At 0° and unit ionic strength k1 is about 0.067 M-1 sec.-1 and k2 is about 0.33 M-1 sec.-1. Also k2 = 4.054 × 104T exp ( -9400/RT). About 65% of the over-all oxidation-reduction reaction involves the intermediate; the rest goes directly to the final products, probably by way of an outer-sphere activated complex.
- Newton,Baker
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p. 569 - 573
(2008/10/08)
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- The dimesitylenevanadium(I) cation
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Hexacarbonylvanadium oxidizes dimesitylenevanadium(0) to the [V(C6H3(CH3)3)2] + cation, the compound [V(C6H3- (CH3)3)2][V(CO)6/s
- Calderazzo, Fausto
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p. 810 - 814
(2008/10/08)
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