Swapan K. Saha

Electron exchange and transfer reactions of heteropoly oxometalates

A. Introduction B. Electron exchange in dodecatungstocobaltate(II/III) and dodecatungstocuprate(I/II) C. Electron transfer reactions of dodecatungstocobaltate(II1) (i) Reactions with inorganic reagents (ii) Reactions with thiols and related species (iii) Reactions with carboxylates, cc-hydroxycarboxylates and amine-N-polycarboxylates (iv) Reactions with carbonyl compounds and esters (v) Reactions with carbohydrates (vi) Reactions with substituted alkyl aromatic compounds (vii) Miscellaneous reactions D. Electron transfer reactions of dodecatungstocobaltate(I1) (i) Reactions of [CO”W,,O,,]~with peroxodisulphate and periodate (ii) Reactions of [Co”W,,O,,]‘~ and [CO”W,,O,,]~with polyhalogenated alkanes E. Electron transfer reactions of dodecamolybdocerate(IV) F. Electron transfer reactions of nonamolybdonickelate(IV) and nonamolybdomanganate(IV) G. Conclusions Note added in proof References . . . .._..._ _........__._.......__.....

Kinetics and mechanism of the reduction of 12-tungstocobaltate(III) by formate. Catalysis by alkali metal ions through outersphere bridging

Abstract The kinetics of the reduction of 12-tungstocobaltate(III) by the molecular and anionic forms of formic acid follow the rate laws -d[Co III M]/d t = 2 k 1 [Co III M] 2 [HCOOH] and -d[Co III M]/d t = 2 k 2 - [HCOO − ] [Co III M] 2 respectively at a constant alkali metal ion concentration. Although the low pH rate ( k 1 ) remains uninfluenced by the alkali cations, the magnitude of the apparent third-order rate constant ( k 2 ) for the oxidation of formate ion largely depends on both the nature and the concentration of the cation present. The cation catalytic order, K + > Na + >Li + , has been successfully explained with the help of the polarizability concept. A first-order dependence of k 2 on [K + ] but second-order dependence on [Na + ] and [Li + ] is observed. These cation assisted paths have been studied thoroughly and the rate constants for these cation accelerated paths along with that for the spontaneous process have been evaluated. Activation parameters corresponding to the rate constants of the spontaneous and sodium ion assisted paths have been determined. A cation bridged outersphere electron transfer mechanism with the generation of free radicals is suggested.

Kinetics and mechanism of the oxidation of iminodiacetate, nitrilotriacetate and ethylenediaminetetraacetate by trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III) in aqueous media

Kinetic studies of the oxidation of iminodiacetate (ida), nitrilotriacetate (nta) and ethylene-diaminetetraacetate (edta) by trans-cyclohexane-1,2-diamine-N,N,N′,N′-tetraacetatomanganate(III), [MnIII(cdta)]–, have been made in aqueous solution in the range pH 3.0–10.0 with varying reductant concentrations at constant ionic strength, I= 0.50 mol dm–3(NaClO4), and temperature, 30°C. All the reactions are first order both in complex and reductant concentration, and follow the general rate law –d[MnIII]/dt=kobs[MnIII]=(kd+ks[R])[MnIII], where kd denotes the autodecomposition rate of the complex, ks the electron-transfer rate and R is the reductant irrespective of the nature and type of the reacting species. The complex [MnIII(cdta)]– showed an interesting behaviour in acidic and alkaline media. For ida oxidation both the aqua- and hydroxo-forms of the complex are reactive and an inner-sphere mechanism has been proposed. However, for nta and edta oxidations, the aqua form is the sole reacting species and an outer-sphere mechanism has been proposed. The rate parameters and proton equilibrium constants of the complex and reductants were obtained by fitting of the experimental data by appropriate rate equations using computer-fit programs. Thus the reactivities of all the species of the polycarboxylates available in the reaction solutions have been evaluated individually. The reactivity orders are Hida– < ida2–, H2nta– < Hnta2– < nta3– and H3edta– < H2edta2– < Hedta3– < edta4–.

Kinetics and mechanism of the reduction of dodecatungstocobaltate (III) by D-fructose, D-glucose, and D-mannose: comparison between keto- and aldohexoses

The kinetics of reduction of dodecatungstocobaltate (III) by D-fructose, D-glucose, and D-mannose in aqueous media have been investigated. A pseudo-zero-order rate was obtained with the ketose and the reaction was acid-dependent. On the contrary, pseudo-second-order rates were obtained for the aldose reactions. The reactions were catalysed by metal ions in the supporting electrolyte; the cation catalytic order K+ > Na+ Li+ observed for aldose oxidation is explained on the basis of electrostatic considerations. A mechanism for the ketose oxidation is offered based on keto–enol tautomerism; the aldose oxidations are explained in terms of a transition state formed via ion-induced dipole bridging.

Kinetics and mechanism of the reduction of dodecatungstocobaltate(III) by α-hydroxyacids in aqueous acid solution. Evidence for alkali metal ion catalysis

SummaryThe kinetics of the redox reaction of dodecatungstocobaltate(III) [abbreviated as CoIIIW] with three α-hydroxy-carboxylic acids:e.g., mandelic, glycolic and lactic acid, were investigated in the 0–5.4 pH range where the complex remains stable and does not decompose. All reactions are first-order both in oxidant and reductant over the experimental pH range. A specific metal ion effect has been observed. Oxidations of the molecular form of the three acids and the anionic form of mandelic acid are catalysed by alkali metal ions and follow a general rate law:

Kinetics and mechanism of the reduction of dodecatungstocobaltate(III) by oxalate. The catalytic role of alkali metal ions through outer-sphere bridging

- d[Co^{III} W]/dt = 2(k_s + k_c [M^ + ])[R][Co^{III} W]

Kinetics and mechanism of the oxidation of some cyclic and acyclic ketones by dodecatungstocobaltate(III)—A comparative study

where ks and kc account for the spontaneous and catalysed paths respectively, M+ is either an alkali metal ion (Li+, Na+ or K+) and [R], the reductant concentration. The oxidation of lactate and glycolate ions occurs only in the presence of alkali cations. The cation catalytic order: K+>Na+>Li+ observed in this study is in accord with the polarizability of the cations. An outer-sphere alkali cation-bridged mechanism is suggested for the electron transfer step.

Kinetics and mechanism of the alkali metal ions promoted electron transfer between 12-tungstocobaltate(III) and citric acid

The reaction between dodecatungstocobaltate(III) and sulphite has been studied in the range pH 1.50–3.50 at 25 °C and l= 0.5 mol dm–3(NaClO4). The reaction stoicheiometry has been found to be 1 : 1 in the presence of excess of sulphite and the corresponding reaction product obtained is dithionate. However, when the complex: sulphur (IV) mole ratio is > 1.5:1, sulphate is also obtained together with dithionate and their amounts are dependent on the mole ratio of the reactants. The stoicheiometry [SIV]consumed:[complex]consumed varies in the range 0.7–1.0:1 on a par with the sulphate: dithionate ratio. The kinetics of the reaction path which gives a 1:1 stoicheiometry follows the rate law (i). A mechanism has been proposed, considering k0 and k1′–d[complex]/dt=(k0+k1′[H+]–1+k2[HSO3–])[complex][HSO3–](i) paths to correspond to the reactions between the cobalt(III) complex with HSO3– and SO32– respectively. The k2 path is assigned to the reaction of the complex with S2O52– or that of sulphur(IV) with an intermediate formed by the reaction between the complex and one sulphur(IV) species. The observed specific alkali-metal-ion catalysis is attributed to the formation of bridging by the alkali-metal ion between the two negatively charged reactants, facilitating the electron-transfer process.