Kinetics and mechanism of an unique hydrolysis reaction of (hexafluoroacetylacetonato)bis(ethylenediamine)cobalt(III) in aqueous solution

Summary Addition of base to the title complex results in the rapid reversible formation of the hydrolysed species Co(en)₂-(hfac · OH)⁺ in which the coordinated hexafluoroacetylacetonato ligand contains a hydroxyl group on the carbonyl carbon atom. The kinetics of both the forward hydrolysis and reverse acidolysis reactions were followed spectrophotometrically using stopped-flow and T-jump techniques. The corresponding rate constants areca. 3×106 and 1×10⁸ M^–1s^–1, respectively, for various buffer systems at 25 °C and ionic strength 1.0 M. A combination of the kinetic and equilibrium data enables the estimation of the uncatalyzed (spontaneous) forward and reverse reaction components. The results are discussed with reference to similar data reported for the hydrolysis and reverse acidolysis reactions of the uncoordinated acetylacetonato ligand.     

Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

The kinetics and mechanisms of the oxidation of I (-) and Br (-) by trans-[Ru (VI)(N 2O 2)(O) 2] (2+) have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru (VI)(N 2O 2)(O) 2] (2+) + 3X (-) + 2H (+) --> trans-[Ru (IV)(N 2O 2)(O)(OH 2)] (2+) + X 3 (-) (X = Br, I). In the oxidation of I (-) the I 3 (-)is produced in two distinct phases. The first phase produces 45% of I 3 (-) with the rate law d[I 3 (-)]/dt = ( k a + k b[H (+)])[Ru (VI)][I (-)]. The remaining I 3 (-) is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I (-)], [H (+)], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru (VI) and I (-), which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru (IV)(N 2O 2)(O)(OHI)] (2+), and a one electron transfer to give [Ru (V)(N 2O 2)(O)(OH)] (2+) and I (*). [Ru (V)(N 2O 2)(O)(OH)] (2+) is a stronger oxidant than [Ru (VI)(N 2O 2)(O) 2] (2+) and will rapidly oxidize another I (-) to I (*). In the second phase the [Ru (IV)(N 2O 2)(O)(OHI)] (2+) undergoes rate-limiting aquation to produce HOI which reacts rapidly with I (-) to produce I 2. In the oxidation of Br (-) the rate law is -d[Ru (VI)]/d t = {( k a2 + k b2[H (+)]) + ( k a3 + k b3[H (+)]) [Br (-)]}[Ru (VI)][Br (-)]. At 298.0 K and I = 0.1 M, k a2 = (2.03 +/- 0.03) x 10 (-2) M (-1) s (-1), k b2 = (1.50 +/- 0.07) x 10 (-1) M (-2) s (-1), k a3 = (7.22 +/- 2.19) x 10 (-1) M (-2) s (-1) and k b3 = (4.85 +/- 0.04) x 10 (2) M (-3) s (-1). The proposed mechanism involves initial oxygen atom transfer from trans-[Ru (VI)(N 2O 2)(O) 2] (2+) to Br (-) to give trans-[Ru (IV)(N 2O 2)(O)(OBr)] (+), which then undergoes parallel aquation and oxidation of Br (-), and both reactions are acid-catalyzed.     

Kinetics and mechanism of the redox reaction between malachite green and iron(III) in aqueous and micellar media

The kinetics of the oxidation of malachite green (MG⁺) by Fe(III) were investigated spectrophotometrically by monitoring the absorbance change at 618 nm in aqueous and micellar media at a temperature range 20–40 °C; I = 0.10 mol dm^?3 for [H⁺] range (2.50–15.00) × 10^?4 mol dm^?3. The rate of reaction increases with increasing [H⁺]. The reaction was carried out under pseudo-first-order conditions by taking the [Fe(III)] (>10-fold) the [MG⁺]. A mechanism of the reaction between malachite green and Fe(III) is proposed, and the rate equation derived from the mechanism was consistent with the experimental rate law as follows: Rate = (k ₄ + K ₁ k ₅[H⁺]) [MG⁺][Fe(III)]. The effect of surfactants, such as cetyltrimethylammonium bromide (CTAB, a cationic surfactant) and sodium dodecylsulfate (SDS, an anionic surfactant), on the reaction rate has been studied. CTAB has no effect on the rate of reaction while SDS inhibits it. Also, the effect of ligands on the reaction rate has been investigated. It is proposed that electron transfer proceeds through an outer-sphere mechanism. The enthalpy and the entropy of the activation were calculated using the transition state theory equation.     

Kinetics of Fe(II) reduction of cis‐halogeno(dodecylamine) bis(ethylenediamine)‐ cobalt(III) complex ion in aqueous solutions

Kinetics of reduction of the surfactant complex ions, cis‐chloro/bromo (dodecylamine)bis(ethylenediamine)cobalt(III) by iron(II) in aqueous solution was studied at 303, 308, and 313 K by spectrophotometry method under pseudo‐first‐order conditions, using an excess of the reductant. The second‐order rate constant remains constant below critical micelle concentration (cmc), but increases with cobalt(III) concentration above cmc, and the presence of aggregation of the complex itself alters the reaction rate. The rate of reaction was not affected by the added [H⁺]. Variation of ionic strength (μ) influences the reaction rate. Activation and thermodynamic parameters have been computed. It is suggested that the reaction of Fe²⁺ (aq) with cobalt(III) complex proceeds by the inner‐sphere mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 98–105, 2006     

Kinetics of aquation of cis-aquabromo-bis(ethylenediamine) complexes of cobalt(III) and chromium(III) in binary aqueous mixtures

Summary The kinetics of aquation of cis-[Co(en)₂(H₂O)Br]²⁺ and cis-[Cr(en)₂(H₂O)Br]²⁺ (en = ethylenediamine) were investigated in aqueous mixtures of MeOH, EtOH, i-PrOH and t-BuOH. The values of transfer functions corresponding to the transfer of reactants and activated complex from water to the solvent mixtures were evaluated from kinetic measurements and from solubilities of the complex salt. Analysis of the solvent effect confirmed a common I_d mechanism for the aquation of the Co^III and Cr^III complexes.     

Kinetics,isotherm and thermodynamics for Cr(III) and Cr(VI) adsorption from aqueous solutions by crystalline hydrous titanium oxide

The synthetic crystalline hydrous titanium(IV) oxide (CHTO), an anatase variety and thermally stable up to 300 °C, has been used for adsorption of Cr(III) and Cr(VI) from the aqueous solutions, the optimum pH-values of which are 5.0 and 1.5, respectively. The kinetic data correspond very well to the pseudo-second order equation. The rates of adsorption are controlled by the film (boundary layer) diffusion, and increase with increasing temperature. The equilibrium data describe very well the Langmuir, Redlich–Peterson, and Toth isotherms. The monolayer adsorption capacities are high, and increased with increasing temperature. The evaluated ΔG^° (kJ · mol^?1) and ΔH^° (kJ · mol^?1) indicate the spontaneous and endothermic nature of the reactions. The adsorptions occur with increase in entropy (ΔS^° = positive), and the mean free energy (E_DR) values obtained by analysis of equilibrium data with Dubinin–Radushkevick equation indicate the ion-exchange mechanism for Cr(III) and Cr(VI)-adsorptions.     

Kinetics of the oxidation of diaqua(nitrilotriacetato)-chromium(III) by periodate in aqueous solutions

Summary The kinetics of oxidation of [Cr^III(nta)(H₂O)₂] (nta is nitrilotriacetate) by periodate obey the rate law d[Cr^VI]/dt=(k₂[IO ₄ ^– ]+k₃[IO ₄ ^– ]²)[Cr^III(nta)(H₂O)₂] under fixed conditions. The activation parameters are reported and we propose that electron-transfer proceeds via an inner-sphere mechanism.     

Kinetics and mechanism of oxidation of tris(ethylenediamine)-chromium(III) by N-bromosuccinimide

Summary In aqueous solutions, [Cr(en)₃]³⁺ aquates to [Cr(en)₂-(H₂O) ₂]³⁺. A kinetic study of the oxidation of [Cr(en)₃]³⁺ by N-bromosuccinimide (NBS) in aqueous solutions and water-alcohol solvent mixtures was performed. The reaction is first order with respect to both total [Cr^III] and [NBS]. The rate is inversely dependent upon [H⁺] in the 7.0–7.9 pH range, and varies with the co-solvent according to the order: MeOH > EtOH > PrOH. An appropriate mechanism, in which the deprotonated [Cr(en)₂(OH)(H₂O)]²⁺ is the reactive species, is suggested. Thermodynamic activation parameters have been calculated.Abstracted from the PhD thesis (Ain Shams University) of A. E.- D. M. Abdel-Hady.     

Conductance behavior of some tris(ethylenediamine)cobalt(III) complexes and their ion association in aqueous solutions

The conductance behavior of some tris(ethylenediamine)cobalt(III) complexes was studied in dilute aqueous solutions at 25°C to investigate the ion-pair formation. The thermodynamic formation constants of the ion pairs [Co(en)₃]³⁺·X^– are 28 (chloride), 28 (bromide), 19 (nitrate), and 15 (perchlorate). These values were compared with theoretical values calculated by using Bjerrum's theory of ion association. The formation constant of [Co(en)₃]³⁺·Cl^– was larger than that obtained from the spectrophotometric measurement in solutions containing perchlorate ion. This difference in the formation constants was explained by considering the contribution of ion association of the complex cation with perchlorate ion.     

Kinetics and mechanisms of the oxidation of the octacyanoniobate(III)ion by oxyanions in alkaline aqueous media

Summary The kinetics and mechanisms of the oxidation of Nb(CN) _inf8 ^sup5– by the oxyanions S₂O _inf8 ^sup2– , BrO _inf3 ^sup– , and IO _inf4 ^sup– have been investigated in alkaline aqueous media (pH 12). The second-order rate constant for the electron transfer reaction between Nb(CN) _inf8 ^sup5– and S₂O _inf8 ^sup2– at 25.0 °C, I = 0.36m (K⁺), is 11.1± 0.3 m ^–1 s ^–1 with H = 30 ± 2kJmol^–1 and S = - 125 + 7JK^–1 mol^–1. The rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by BrO _inf3 ^sup– at 25.0 °C, I = 0.20m (Na⁺), is 2.39 ± 0.08m ^–1 s ^–1 with H = 28 ± 2kJmol^–1 and S = -139 ± 7JK^–1mol^–1. The oxidation of Nb(CN) _inf8 ^sup5– by IO _inf4 ^sup– proceeds by two parallel pathways involving the monomeric IO _inf4 ^sup– ion and the hydrated dimer H₂I₂O _inf10 ^sup4– . The second-order rate constant for the oxidation of Nb(CN) _inf8 ^sup5– by monomeric IO _inf4 ^sup– at 5.0 °C, I = 0.050m (Na⁺), is (3.3 ± 0.6) × 10³ m ^–1 s ^–1 with H = 75 ± 6 kJ mol^–1 and S = 94 ± 15 J K^–1 mol^–1, while the rate constant for the oxidation by H₂I₂O _inf10 ^sup4– is (1.8 ± 0.1) × 10³ m ^–1 s ^–1 with H = 97 ± 5 kJ mol^–1 and S = 166 ± 16 J K^–1 mol^–1 under the same reaction conditions. The rate constants for each of the oxidants employed display specific cation catalysis with the order of increasing rate constants: Li⁺ < Na+ < NH _inf4 ^sup+ < K⁺ < Rb⁺ < Cs⁺, in the same direction as the electronic polarizability of the cations. The results are discussed in terms of the outer-sphere electron-transfer processes and compared with the corresponding data and mechanisms reported for other metal-cyano reductants.     

Kinetics of oxidation ofcis-, andtrans-chlorobis(ethylenediamine)isothiocyanatocobalt(III)ion with peroxodisulphate in aqueous alcoholic mixtures

Summary The effect of organic cosolvent on the rate constants and activation parameters of oxidation of coordinativelybound thiocyanate with peroxodisulphate was investigated in mixtures of water with methanol,i-propanol andt-butanol. The thermodynamic transfer functions, corresponding to the transfer of reactants and activated complex from water to the solvent mixtures were evaluated from kinetic measurements and from the solubilities of corresponding salts. The experimental results are interpreted in terms of the solvation of both the initial state and the activated complex.     

Coprecipitation of titanium(IV) and Iron(III) complex salts from peroxide-fluoride solutions

Because the salts (NH₄)₃TiO₂F₅ and (NH₄)₃FeF₆ are isostructural, precipitation of the titanium peroxy salt from the (NH₄)₂TiF₆-(NH₄)₃FeF₆-NH₄F-H₂O₂ system is accompanied by crystal-chemical substitution of ions. As a result of partial peroxolysis, the iron salt coprecipitates with ammonium peroxypentafluorotitanate. If the degree of precipitation is higher than 20%, TiO₂ samples of high whiteness can be prepared from the hydrolysis product isolated from such solutions, after annealing at 850°C.     

Stopped‐flow investigation of the reaction of hexabromoiridate(III) with bromine in aqueous solution

The reaction of hexabromoiridate(III) with bromine to form hexabromoiridate(IV) and bromide ion was studied using a Durrum stopped‐flow spectrophotometer. Special precautions were taken while studying this reaction. All of the experiments were carried out within 3 min after preparation of the hexabromoiridate solution throughout this investigation in order to limit the amount of conversion of hexabromoiridate(III) to aquapentabromoiridate(III) before reaction with bromine. Very dilute solutions were used because of the rapidity of the reaction. The rate constant for the reaction at 24 ± 1°C was determined to be (3.2 ± 0.4) × 10³ L mol^?1 s^?1. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 623–626, 2004     

Kinetics and mechanism of oxidation of cis-diaquabis(glycinato)chromium(III) by periodate ion in aqueous solutions

The kinetics of oxidation of cis-[Cr^III(gly)₂(H₂O)₂]⁺ (gly = glycinate) by $ {\text{IO}}_{ 4}^{ - } $ has been studied in aqueous solutions. The reaction is first order in the chromium(III) complex concentration. The pseudo-first-order rate constant, k _obs, showed a small change with increasing $ \left[ {{\text{IO}}_{ 4}^{ - } } \right] $ . The pseudo-first-order rate constant, k _obs, increased with increasing pH, indicating that the hydroxo form of the chromium(III) complex is the reactive species. The reaction has been found to obey the following rate law: $ {\text{Rate}} = 2k^{\text{et}} K_{ 3} K_{ 4} \left[ {{\text{Cr}}\left( {\text{III}} \right)} \right]_{t} \left[ {{\text{IO}}_{ 4}^{ - } } \right]/\left\{ {\left[ {{\text{H}}^{ + } } \right] + K_{ 3} + K_{ 3} K_{ 4} \left[ {{\text{IO}}_{ 4}^{ - } } \right]} \right\} $ . Values of the intramolecular electron transfer constant, k ^et, the first deprotonation constant of cis-[Cr^III(gly)₂(H₂O)₂]⁺, K ₃ and the equilibrium formation constant between cis-[Cr^III(gly)₂(H₂O)(OH)] and $ {\text{IO}}_{ 4}^{ - } $ , K ₄, have been determined. An inner-sphere mechanism has been proposed for the oxidation process. The thermodynamic activation parameters of the processes involved are reported.     

Sorption of thallium(III) ions from aqueous solutions using titanium dioxide nanoparticles

Titanium dioxide nanoparticles (NPs) were employed for the sorption of Tl(III) ions from aqueous solution. The process was studied in detail by varying the sorption time, pH, Tl(III) concentration, temperature, and amount of sorbent. The sorption was found to be fast and to reach equilibrium within 2 min, to be less efficient at low pH values, and to increase with pH and temperature. The sorption fits the Langmuir equation and follows a pseudo second order model. The mean energy of the sorption is approximately 15 kJ mol^?1 as calculated from the Dubinin–Radushkevich isotherm. The thermodynamic parameters for the sorption were also determined, and the ΔH ⁰ and ΔG ⁰ values indicate endothermic behavior.     

Kinetics and mechanism of the ligand substitution reaction of aquodiethylenetriaminepentaacetatoruthenate (III) in aqueous solution

The kinetics of ligand substitution reactions of [Ru(H₂dtpa) (H₂O)] (2) (H₂dtpa=diprotonated diethylenetriaminepentaacetic acid) were studied as a function of ligand (L) concentration, pH (2.5–8.0) and temperature (30–45 °C) at 0.2 M ionic strength. The equilibrium constants for the formation of mixed ligand complex [Ru^III(dtpa) (L)] (L=2-mercaptopyrimidine, cysteine) and the distribution of various species in solution in the pH range of 2.5–8.0 were computed from potentiometric results. [Ru(H₂dtpa) (H₂O)] ( H₂dtpa= ) , pH (2,5–8,0) (30–45°C) 0,2 M. [Ru^III(dtpa) (L)] ( L=2-, ) pH=2,5–8,0.