Intermediate and ion-pair formation in the outer-sphere reactions between azido-pentacyanocobaltate(III) and iron(II) polypyridyl complexes in aqueous medium

The kinetics of the reactions between azido-pentacyanocobaltate(III), Co(CN)₅N₃ ³⁻, and iron(II) polypyridyl complexes, Fe(LL)₃ ²⁺ (LL = bipy, phen), have been studied in both neutral and acidic aqueous solutions at I = 0.1 mol dm⁻³ NaCl. The reactions were carried out under pseudo-first-order conditions in which the concentration of Fe(LL)₃ ²⁺ was kept constant, and the second-order rate constants obtained for the reactions at 35 °C were within the range of 0.156–0.219 dm³ mol⁻¹ s⁻¹ for LL = bipy and 0.090–0.118 dm³ mol⁻¹ s⁻¹ for LL = phen. Activation parameters were measured for these systems. The dependence of reaction rates on acid was studied in the range [H⁺] = 0.001–0.008 mol dm⁻³. The reaction in acid medium shows interesting kinetics. Two reactive species were identified in acid medium, namely, the protonated cobalt complex and the azido-bridged binuclear complex. The electron-transfer process is proposed to go by mixed outer- and inner-sphere mechanisms in acid medium, in which electron transfer through the bridged inner-sphere complex (k ₅) is slower than through the outer-sphere path (k ₄). Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Kinetics and mechanism of the oxidation of iron(II) ion by chlorine dioxide in aqueous solution

The mechanism by which an excess of iron(II) ion reacts with aqueous chlorine dioxide to produce iron(III) ion and chloride ion has been determined. The reaction proceeds via the formation of chlorite ion, which in turn reacts with additional iron(II) to produce the observed products. The first step of the process, the reduction of chlorine dioxide to chlorite ion, is fast compared to the subsequent reduction of chlorite by iron(II). The overall stoichiometry is The rate is independent of pH over the range from 3.5 to 7.5, but the reaction is assisted by the presence of acetate ion. Thus the rate law is given by At an ionic strength of 2.0 M and at 25°C, k_u = (3.9 ± 0.1) × 10³ L mol^?1 s^?1 and k_c = (6 ± 1) × 10⁴ L mol^?1 s^?1. The formation constant for the acetatoiron(II) complex, K_f, at an ionic strength of 2.0 M and 25°C was found to be (4.8 ± 0.8) × 10^?2 L mol^?1. The activation parameters for the reaction were determined and compared to those for iron(II) ion reacting directly with chlorite ion. At 0.1 M ionic strength, the activation parameters for the two reactions were found to be identical within experimental error. The values of ΔH? and ΔS? are 64 ± 3 kJ mol^?1 and + 40 ± 10 J K^?1 mol^?1 respectively. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 554–565, 2004     

Functionalisation of carbon dioxide by an iron(II) complex

Addition of carbon dioxide to trans-Fe(dmpe)2(SCHNEt)H 2 affords the iminium carboxylate trans-Fe(dmpe)2(SCHN+(Et)CO2-)H 4, which rearranges to the ferracyle cis-Fe(dmpe)2(SCH2N(Et)C(O)O-kappa S,O) 5.     

Ion-pair formation in the outer-sphere electron transfer reactions between tris-(1, 10) phenanthroline iron(II) and the halopentacyanocobaltate(III) complexes in aqueous acidic solutions

The kinetics of the reactions between Fe(phen) ₃ ²⁺ [phen = tris–(1,10) phenanthroline] and Co(CN)₅X³⁻ (X = Cl, Br or I) have been investigated in aqueous acidic solutions at I = 0.1 mol dm⁻³ (NaCl/HCl). The reactions were carried out at a fixed acid concentration ([H⁺] = 0.01 mol dm⁻³) and the second-order rate constants for the reactions at 25 °C were within the range of (0.151–1.117) dm³ mol⁻¹ s⁻¹. Ion-pair constants K _ip for these reactions, taking into consideration the protonation of the cobalt complexes, were 5.19 × 10⁴, 3.00 × 10² and 4.02 × 10⁴ mol⁻¹ dm⁻³ for X = Cl, Br and I, respectively. Activation parameters measured for these systems were as follows: ΔH* (kJ K⁻¹ mol⁻¹) = 94.3 ± 0.6, 97.3 ± 1.0 and 109.1 ± 0.4; ΔS* (J K⁻¹) = 69.1 ± 1.9, 74.9 ± 3.2 and 112.3 ± 1.3; ΔG* (kJ) = 73.7 ± 0.6, 75.0 ± 1.0 and 75.7 ± 0.4; E _a (kJ) = 96.9 ± 0.3, 99.8 ± 0.4, and 122.9 ± 0.3; A (dm³ mol⁻¹ s⁻¹) = (7.079 ± 0.035) × 10¹⁶, (1.413 ± 0.011) × 10¹⁷, and (9.772 ± 0.027) × 10²⁰ for X = Cl, Br, and I respectively. An outer – sphere mechanism is proposed for all the reactions.     

Simultaneous determination of iron(II) and iron(III) in aqueous solution by kinetic spectrophotometry with tiron

A kinetic spectrophotometric method that requires no prior measurement of rate constants is developed for the simultaneous determination of iron(II) and iron(III). The method is based on the aerial oxidation of iron(II) in the presence of tiron and acetate ions. The iron(III) formed is subsequently complexed with tiron and the absorbance/time relation is evaluated. The concentrations of iron(III) and iron(II) are obtained from the absorbance values at the start and at equilibrium, respectively, calculated by non-linear least-squares fitting. A linear calibration graph is obtained up to 12 μg ml^?1 iron(II)/iron(III). The method is applied to iron-rich ground water.     

Structure of iron (II) and cobalt (II) bromide complexes in aqueous solution by X-ray diffraction analysis

Structures of bromo-metal complexes in concentrated aqueous solutions of FeBr₂ and of CoBr₂ were investigated by X-ray diffraction analysis. The complexes possess an octahedral geometry coordinating Br^– along with H₂O ligands. The frequency factors of metal-Br contacts per one atom of metal were 0.325 for the 2.7M (mol-dm^–3) and 0.747 for the 4.5M FeBr₂ solutions, and 0.280 for the 2.8M and 0.595 for the 4.3M CoBr₂ solutions. The frequency factors suggested that the tendency of metal ions to forming monobromo complexes is in the order, Fe>Co>Ni相似文献   

Thermal and photochemical reduction of aqueous chlorine by ruthenium(II) polypyridyl complexes

Studies are reported on the reactions of aqueous chlorine with a series of substitution-inert, one-electron metal-complex reductants, which includes [Ru(bpy)3]2+, [Ru(4,4'-Me2bpy)3]2+, [Ru(4,7-Me2phen)3]2+, [Ru(terpy)2]2+, and [Fe(3,4,7,8-Me4phen)3]2+. The reactions were studied by spectrophotometry at 25 degrees C in acidic chloride media at mu = 0.3 M. In general the reactions have the stoichiometry 2[ML3]2+ + Cl2-->2[ML3]3+ + 2Cl-. In the case of [Ru(bpy)3]2+, the reaction is quite photosensitive; the thermal reaction is so slow as to be practically immeasurable. The reactions of [Ru(4,4'-Me2bpy)3]2+ and [Ru(4,7-Me2phen)3]2+ are also highly photosensitive, giving pseudo-first-order rate constants that depend on the monochromator slit width in a stopped-flow instrument; however, the thermal rates are fast enough that they can be obtained by extrapolation of kobs to zero slit width. The reactions of [Ru(terpy)2]2+ and [Fe(3,4,7,8-Me4phen)3]2+ show no appreciable photosensitivity, allowing direct determination of their thermal rate laws. From the kinetic effects of pH, [Cl2]tot, and [Cl-] it is evident that all of the thermal rate laws have a first-order dependence on [ML3]2+ and on [Cl2]. The second-order rate constants decrease as Eo for the complex increases, consistent with the predictions of Marcus theory for an outer-sphere electron-transfer mechanism. Quantum yields at 460 nm for the reactions of [Ru(4,4'-Me2bpy)3]2+ and [Ru(4,7-Me2phen)3]2+ exceed 0.1 and show a dependence on [Cl2] indicative of competition among spontaneous decay of *Ru, nonreactive quenching by Cl2, and reactive quenching by Cl2.     

Some quantum chemical aspects on outer-sphere electron-transfer reactions: the U(V)/Fe(III)-U(VI)/Fe(II) system

The activation energy and rate constant of U(V)-Fe(III) to U(VI)-Fe(II) outer-sphere electron-transfer reaction was studied using Marcus model. Experimental values were used for Gibbs energy change of the reaction, and energy surfaces were calculated by quantum chemical methods. The calculated rate constant was in reasonable accord with experimental value.     

Evidence for absence of an inverted region for Ru(III)-Ti(III) outer-sphere electron-transfer reaction

A lower limit of 10⁸ M^?1 s^?1 was obtained for the 2nd-order rate constant at 25°C for the reduction of Ru(bpy)₃³⁺ by Ti(III) in an aqueous medium of ionic strength 1.0 M and pH range 0–2. The theoretical implications of this result are discussed.     

Emission quenching of binuclear pyrophosphito platinum(II) complexes in aqueous solution by sulphur dioxide. Spectroscopic measurements on sulphur dioxide addition and chromous ion reduction

The emission intensity at 517 nm from Pt₂(pop)₄⁴⁻ (pop = P₂O₅H₂²⁻) is quenched by the addition of sulphur dioxide. The sulphur dioxide coordinates at the axial platinum(II) sites by a η¹-SO₂ bond. This coordination is supported by ³¹p NMR and Raman spectroscopy of aqueous solutions. The electronic spectrum of a sulphur dioxide saturated solution of Pt₂(pop)₄⁴⁻ shows an absorption at 428.5nm ( = 4.1 × 10⁴). From the decrease in the chromophore for uncomplexed Pt₂(pop)₄⁴⁻ the equilibrium constant for SO₂ binding is estimated to be 1.74 M₂l⁻². The effect of adding different quenchers to aqueous solutions of Pt_2(pop)4⁴⁻ is discussed. The compound Pt₂(pop)₄⁴⁻ will undergo 2-electron reduction with chromous ion.     

The chemistry of adsorbed Cu(II) and Mn(II) in aqueous titanium dioxide suspensions

We studied the adsorption behavior of Cu(II) and Mn(II) on the surface of titanium dioxide over the pH range from 2.0 to 11.5. The titanium dioxide we used in these experiments was prepared by hydrolyzing TiCl₄ and had a surface area of 113.7 m² g⁻¹. All suspensions, which were 9.04 × 10⁻³ M in NaClO₄, contained 20 m² liter⁻¹ of oxide surface and divalent metal ion concentrations sufficient (at full adsorption from solution) to cover the available surface with one-half, one, and four layers of close-packed, hydrated ions. Both divalent ions began adsorption below titanium dioxide's isoelectric point (pH = 6.2). Cu²⁺ adsorption was accompanied by net OH⁻ uptake from solution and it was inferred that the titania surface also provided OH⁻ for Cu²⁺ adsorption. ESR spectra demonstrate the coexistence of two distinct forms adopted by these metal ions on the surface. A portion of the adsorbed metal ions occupies sites magnetically isolated one from another, as evidenced by the paramagnetic behavior of this form. The majority of the metal ions, however, exist in hydrous-metal-ion clusters in which spin-exchange coupling of the electron dipoles determines the magnetic behavior. Electrophoretic mobility measurements indicate that ions adsorbed at isolated sites exert a stronger influence on the electrophoretically measured charge of the suspension particles than ions in clusters. Even though these experiments were performed in the absence of oxygen, we observed the oxidation of a limited amount of the Mn(II) on the surface as low as pH = 5. Presumably this occurs as a result of electron transfer between photo-induced electron holes and Mn(II) on the surface.     

Determination of iron(II) ions in aqueous solutions by secondary-ion mass spectrometry

Model samples of aqueous solutions containing iron(II/III) salts have been studied by secondary-ion mass spectrometry. Mass spectra of the samples containing ferrous and ferric ions have been compared. The possibility of using secondary-ion mass spectrometry for the analysis of rain precipitations and samples of atmospheric aerosols has been considered.     

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.     

Copper(II) catalyses the reduction of perchlorate by both formaldehyde and by dihydrogen in aqueous solutions

Abstract

During an effort to synthesize the trans-III-copper(II) complex with 1,4,8,11-tetramethyl-pyro-phosphonate-1,4,8,11-tetra-aza-cyclo-tetradecane, using only perchlorate salts, it was noted that the perchlorate is reduced to chloride. Analysis of the reactions leading to this surprising result points out that Cu(H₂O)₄²⁺ catalyzes the reduction of perchlorate by H₂ and by CH₂O. These reactions are slow at room temperature and ambient pressures. A plausible mechanism, supported by DFT calculations, is proposed pointing out that the role of CuH⁺ under mild conditions cannot be ignored.     

Catalysis by methyltrioxorhenium(VII): reduction of hydronium ions by europium(II) and reduction of perchlorate ions by europium(II) and chromium(II)

The title reactions occur stepwise, the first and fastest being MeReO3 + Eu2+ --> Re(VI) + Eu3+ (k298 = 2.7 x 10(4) L mol(-1) s(-1)), followed by rapid reduction of Re(VI) by Eu2+ to MeReO2. The latter species is reduced by a third Eu2+ to Re(IV), a metastable species characterized by an intense charge transfer band, epsilon410 = 910 L mol(-1) cm(-1) at pH 1; the rate constant for its formation is 61.3 L mol(-1) s(-1), independent of [H+]. Yet another reduction step occurs, during which hydrogen is evolved at a rate v = k[Re(IV)][Eu2+][H+](-1), with k = 2.56 s(-1) at mu = 0.33 mol L(-1). The 410 nm Re(IV) species bears no ionic charge on the basis of the kinetic salt effect. We attribute hydrogen evolution to a reaction between H-ReVO and H3O+, where the hydrido complex arises from the unimolecular rearrangement of Re(III)-OH in a reaction that cannot be detected directly. Chromium(II) ions do not evolve H2, despite E(Cr) degrees approximately E(EU) degrees. We attribute this lack of reactivity to the Re(IV) intermediate being captured as [Re(IV)-O-Cr(III)]2+, with both metals having substitutionally inert d3 electronic configurations. Hydrogen evolution occurs in chloride or triflate media; with perchlorate present, MeReO2 reduces perchlorate to chloride, as reported previously [Abu-Omar, M. M.; Espenson, J. H. Inorg. Chem. 1995, 34, 6239-6240].