The kinetics of the oxidation-reduction reaction between uranium(VI) and titanium(III) in HCl solution

The oxidation-reduction reaction between U(VI) and Ti(III) in HCl solution was studied spectrophotometrically. The reaction is second-order at all concentrations of reactants, HCl, ferrous chloride and mannitol used in this work. In 5M HCl the rate constantk increases with increasing Ti(III) concentration, whereas it decreases with increasing U(VI) concentration, with increasing HCl concentration from 1.00M to 7.17M and increases thereafter from 7.17M to 11.79M. The addition of mannitol causes a consistent decrease in the rate of reaction, whereas ferrous chloride has no effect. The activation energy for this oxidation-reduction reaction was 47.90±0.11 kJ·mol^–1. The values of H , G and S were 45.40±0.11 kJ·mol^–1, 72.50±0.17 kJ·mol^–1 and –91.10±0.22J·k^–1·mol^–1, respectively. The mode of reaction is discussed in the light of kinetic results.     

Kinetics and mechanism of the acid-catalyzed hydrolysis of [carbonatoethylenediamine(L)2cobalt(III)]+ ions

The kinetics of acid-catalyzed hydrolysis of the [Co(en)(L)₂(O₂CO)]⁺ ion (L = imidazole, 1-methylimidazole, 2-methylimidazole) follows the rate law –d[complex]/dt = {k ₁ K[H⁺]/(1 + K[H⁺])}[complex] (15–30 or 25–40 °C, [H⁺] = 0.1–1.0 M and I = 1.0 M (NaClO₄)). The reaction course consists of a rapid pre-equilibrium protonation, followed by a rate determining chelate ring opening process and subsequent fast release of the one-end bound carbonato ligand. Kinetic parameters, k ₁ and K, at 25 °C are 5.5 × 10^–2 s^–1, 0.44 M^–1 (ImH), 5.1 × 10^–2 s^–1, 0.54 M^–1 (1-Meim) and 3.8 × 10^–3 s^–1, 0.74 M^–1 (2-MeimH) respectively, and activation parameters for k ₁ are H₁ = 43.7 ± 8.9 kJ mol^–1, S₁ = –123 ± 30 J mol^–1 deg^–1 (ImH), H₁ = 43.1 ± 0.3 kJ mol^–1, S₁ = –125 ± 1 J mol^–1 deg^–1 (1-Meim) and H₁ = 64.2 ± 4.3 kJ mol^–1, S₁ = –77 ± 14 J mol^–1 deg^–1 (2-MeimH). The results are compared with those for similar cobalt(III) complexes.     

Kinetics and mechanism of the interaction of azide with [(H2O)(tap)2RuORu(tap)2-(H2O)]2+ ion at physiological pH

The title reaction has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], pH and temperature at constant ionic strength. At the physiological pH (7.4) the interaction with azide shows two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of N₃ ^–; both processes are [ligand]-dependent. The rate constant for the processes are: k ₁10^–3 s^–1 and k ₂10^–5 s^–1. The activation parameters calculated from Eyring plots are: H ₁ = 14.8 ± 1 kJ mol^–1, S ₁ = –240 ± 3 J K^–1 mol^–1, H ₂ = 44.0 ± 1.5 kJ mol^–1 and S ₂ = –190 ± 4 J K^–1 mol^–1. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for the interaction process. From the temperature dependence of the outersphere association equilibrium constant, the thermodynamic parameters calculated are: H ₁ ⁰ = 4.4 ± 0.9 kJ mol^–1, S ₁ ⁰ = 64 ± 3 J K^–1 mol^–1 and H ₂ ⁰ = 14.2 ± 2.9 kJ mol^–1, S ₂ ⁰ = 90 ± 9 J K^–1 mol^–1, which gives a negative G ⁰ value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex.     

Metal ion catalysed aquation of carboxylatoamine cobalt(III) complexes. Kinetics of copper(II) catalysed aquation ofcis(diformato)bis(ethylenediamine)cobalt(III) ion

Summary The aquation ofcis-[(en)₂Co(CO₂H)₂]⁺ tocis-[(en)₂Co(OH₂)(CO₂H)]²⁺ is catalysed by Cu²⁺ and the rate equation, –d[complex]_t/dt=(k_Cu[Cu²⁺]+k_H [H⁺]) [complex)_T is valid at [Cu²⁺]_T=0.01–0.1, I=0.5 and [HClO₄]=0.005 mol dm^–3. The rate measurements are reported at 30, 35, 40 and 45°C and the rate and activation parameters for the Cu²⁺ and H⁺-catalysed paths are: k_H(35°C)=(2.44±0.09)×10^–2 dm³ mol^–1 s^–1, H=83±13 kJ mol^–1, S=–8±42 JK^–1 mol^–1, k _Cu (35°C)=(3.30±0.09)×10^–3 dm³ mol^–1 s^–1, H=73.2±6.1 kJ mol^–1, S=–55±20 JK^–1 mol^–1. The formate-bridged innersphere binuclear complex,cis-[(en)₂Co{(O₂CH)₂Cu}]³⁺ may be involved as the catalytically active intermediate in the copper(II)-catalysed path, just as the corresponding H⁺-bridged species presumed to be present in the acidcatalysed path.     

Mechanism of the oxidation of L-ascorbic acid by the bis(pyridine-2,6-dicarboxylate)cobaltate(III) ion in aqueous solution

A detailed investigation of the oxidation of L-ascorbic acid (H₂A) by the title complex has been carried out using conventional spectrophotometry at 510 nm, over the ranges: 0.010 [ascorbate] _T 0.045 mol dm^–3, 3.62 pH 5.34, and 12.0 30.0 °C, 0.50 I 1.00 mol dm^–3, and at ionic strength 0.60 mol dm^–3 (NaClO₄). The main reaction products are the bis(pyridine-2,6-dicarboxylate)cobaltate(II) ion and l-dehydroascorbic acid. The reaction rate is dependent on pH and the total ascorbate concentration in a complex manner, i.e., k _obs = (k ₁ K ₁)[ascorbate] _T /(K ₁ + [H⁺]). The second order rate constant, k ₁ [rate constant for the reaction of the cobalt(III) complex and HA^–] at 25.0 °C is 2.31 ± 0.13 mol^–1 dm³ s^–1. H = 30 ± 4 kJ mol^–1 and S = –138 ± 13 J mol^–1 K^–1. K ₁, the dissociation constant for H₂A, was determined as 1.58 × 10^–4 mol dm^–3 at an ionic strength of 0.60 mol dm^–3, while the self exchange rate constant, k ₁₁ for the title complex, was determined as 1.28 × 10^–5 dm³ mol^–1 s^–1. An outer-sphere electron transfer mechanism has been proposed.     

Kinetics of the interaction of aquo-ethylenediaminetetraacetatoruthenate(III) with ferricyanide ion in water

Summary The interaction of aquo-ethylenediaminetetraacetatoruthenate(III) with ferricyanide ion was studied spectrophotometrically as a function of ferricyanide ion concentration, pH (1.5–8.5) and temperature (30–45°C) at ionic strength 0.2 M (NaClO₄). Kinetic and activation parameters (H=27.1±1.75 KJ mol^–1, S=–136.7±5.57 J mol^–1 deg^–1) are consistent with the proposed mechanism.     

Kinetics and mechanism of the reaction betweencis-dichlorobisbipyridineruthenium(II) and nitric acid in water-methanol solutions

Summary The kinetics of the reaction betweencis-dichlorobisbipyridineruthenium(II) and nitric acid have been investigated spectrophotometrically in the 25°–40° range in the presence of 0.03 to 0.2 mol dm^–3 HNO₃. The reaction proceeds with the stepwise formation of monoaqua and diaqua products. Only the formation of the monoaqua intermediate was followed as this species could not be obtained in a pure state. Aquation proceeds through a dissociative process. The second order rate constants are 11.8 (25°), 17.5 (30°); 30.0 (35°) l mol^–1 s^–1. Activation parameters are H 52±3 kJ mol^–1; S–108±8 JK^–1 mol^–1.     

Kinetics and mechanisms of the reactions of thiosulphato-amine complexes of cobalt(III). Part II. Base hydrolysis of the bis(ethylenediamine)thiosulphatocobalt(III) ion

Summary Base hydrolysis of the bis(ethylenediamine)thiosulphatocobalt(III) was investigated spectrophotometrically between 35 and 65 °C and with base concentrations (NaOH) up to 2.0 mol dm^–3. The hydrolysis consists of a one-stage reaction, followed by a slow dechelation step, and then by a fast ligand loss. The reaction is base-dependent. The products of the reaction are an equilibrium mixture ofcis- andtrans-Coen₂ (OH) ₂ ⁺ . Activation parameters for the reaction as determined by the Eyring equation, are H=77.8±4.6 kJ mol^–1 and S=–75±20 JK^–1 mol^–1.     

Chromium(VI), chromium(V) and chromium(IV) oxidation of 12-tungstocobaltate(II)

The reaction between Cr^VI and 12-tungstocobaltate(II) was carried out in 2.0 mol dm^–3 HCl and followed a simple second order rate law. The reaction was catalysed by hydrogen ion due to the formation of active H₂CrO₄ and was inhibited by chloride ion as, in its presence, conversion of the active species into inactive chlorochromate occurs. Chromium(V) and chromium(IV) were generated in situ by the use of Cr^VI—V^IV or Cr^VI—2-ethyl-2-hydroxybutyric acid and Cr^VI—i-PrOH reactions respectively, and the oxidation of 12-tungstocobaltate(II) by these atypical oxidation states, was also studied. The rate constants for the oxidation of 12-tungstocobaltate(II) by Cr^VI, Cr^V and Cr^IV were found to be in the ratio 1:1.2:5.2 respectively. The ionic strength did not affect the reaction, while decrease in the solvent polarity increased the rate of the reaction. The activation parameters were also determined and the values H , G and S were found to be 52.4 ± 6 kJ mol^–1, 100.8 ± 7 kJ mol^–1, –151.7 ± 10 J K^–1 mol^–1 respectively, supporting the mechanism proposed.     

Complexation,solubilities and thermodynamic functions for cerium(III) fluoride-water system

The solubility, solubility product and the thermodynamic functions for the CeF₃–H₂O system have been measured using the radiometric, conductometric and potentiometric techniques. The radiometric values for the solubility and solubility product, the lowest and more acceptable for reasons cited in previous papers, are 3.14·10^–5 M and 2.17·10^–17 respectively. The enthalpy change measured by the conductometric method is almost twice as that obtained by potentiometric method due to abnormal conductances registered at higher temperatures. The average values for H^o and G^o and S^o at 298 K are 53.0±17.4, 91.7±4.0 and –129.7±58.2 KJ·mol^–1 respectively. The positive values for H^o and G^o and the negative value for S^o are indicative of the low solubility of this salt in water. The stability constants for the mono- and difluoride complexes of Ce(III) have been determined potentiometrically using unsaturated solution mixtures of Ce(III) and F^–. These values for CeF⁺ and CeF ₂ ⁺ are 997±98 and (1.03±0.44)·10⁵, respectively. Studies on pH dependence of the solubility shows that the solubility reaches a minimum value at a pH of about 3.2.     

Kinetics of substitution of aqua ligands from cis-diaqua(ethylenediamine)platinum(II) perchlorate by DL-penicillamine in aqueous medium

The kinetics of the interaction of DL-penicillamine with [Pt(en)(H₂O)₂]²⁺ have been studied spectrophotometrically as a function of [Pt(en)(H₂O)₂]²⁺, [DL-penicillamine] and temperature at pH 4.0. The reaction proceeds via rapid outer sphere association complex formation, followed by two slow consecutive steps. The first is the conversion of the aforementioned complex into the inner sphere complex and the second is the slower chelation step whereby another aqua ligand is replaced. The association equilibrium constant (K _E) for the outer sphere complex formation has been evaluated together with rate constants for the two subsequent steps. Activation parameters have been calculated for both steps using the Eyring equation (H ₁ = 46.5 ± 5.0 kJ mol^–1, S ₁ = – 143.0 ± 15.0 J K^–1 mol^–1, H ₂ = 44.3 ± 1.3 kJ mol^–1, S ₂ = –189.0 ± 4.2 J K^–1 mol^–1). The low enthalpy of activation and large negative entropy of activation values indicate an associative mode of activation for both aqua ligand substitution processes.     

NMR study of the sigmatropic [1,3]-B shift in 7,8-dipropyl-7-borabicyclo[4.2.2]deca-2,4,9-triene

Activation parameters of the interconversion of geometric isomers6a and6b were determined by a complete lineshape analysis of the temperature-dependent¹³C NMR spectra of 7,8-dipropyl-7-borabicyclo[4.2.2]deca-2,4,9-triene (6). For the reaction6a 6b, G ₂₉₈ = 52.2±0.1 kJ mol^–1, H = 27.9±0.5 kJ mol^–1, S = –82±8 J mol^–1 K^–1; For the reaction6b 6a, G ₂₉₈ = 52.6±0.1 kJ mol^–1, H = 24.7±0.5 kJ mol^–1, S = –93±10 J mol^–1 K^–1. The interconversion of deuteropyridine complexes9a and9b proceedsvia their dissociation, which indicates that the rearrangement of borane6 occurs according to the [1,3]-B shift mechanism.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2243–2250, September, 1996.     

Kinetics of the displacement of aqua ligands from cis-diaqua(ethylenediamine)-platinum(II)perchlorate by adenosine 5′-monophosphate in aqueous medium

The kinetics of the interaction of adenosine 5-monophosphate (5-AMP) with cis-[Pt(en)(H₂O)₂]²⁺ have been studied spectrophotometrically as a function of [Pt(en)(H₂O)₂]²⁺, [5-AMP] and temperature at pH 4.0, where the substrate complex exists predominantly as the diaqua species. Both N1 and N7 donor sites of 5-AMP are active for coordination to Pt at this pH. Base stacking and metal-induced macrochelate formation of 5-AMP plays a vital role in determining the concentration limit of 5-AMP during kinetics. Substitution occurs in two consecutive steps; both dependent on the 5-AMP concentration. Activation parameters for both steps have been calculated. The low H ₁ (42.76 ± 1.64 kJ mol^–1) and large negative values of S ₁ (–112.1 ± 5.1 J K^–1 mol^–1) as well as H ₂ (58.1 ± 1.4 kJ mol^–1) and S ₂ (–84.2 ± 4.4 J K^–1 mol^–1) indicate associative modes of activation for both ligand substitution processes in the two consecutive steps.     

Complex formation between nickel(II) and 2-(2-aminoethyl) benzimidazole: A kinetic and equilibrium study

Summary The reversible complex formation between 2-(2-aminoethyl) benzimidazole (AEB) and nickel(II) was studied by stopped flow spectrophotometry at I = 0.30 mol dm^–3. Both the neutral and monoprotonated form of AEB reacted to give the NiAEB²⁺ chelate. At 25 °C, the rates and activation parameters for the reactions Ni_II + AEB NiAEB²⁺ and Ni^II + AEBH⁺ NiAEB²⁺ + H⁺ are k _f ^L(dm^–3 mol^–1 s^–1) = (2.17 ± 0.24) × 10³, H (kJ mol^–1) = 40.0 ± 0.8, S (JK^–1 mol^–1) = – 47 ± 3 and k _inff ^pHL (dm³ mol^–1 s^–1) = 33 ± 10, H (kJ mol^–1) = 42.0 ±2.7, S (JK^–1 mol^–1) = – 72 ± 9. The dissociation of NiAEB²⁺ was acid catalysed and k _obs for this process increased linearly with [H⁺] in the 0.01–0.15 mol dm^–3 (10–30 °C) range with k _H(dm³ mol^–1s^–1) (25 °C) = 329 ± 6, H (kJ mol^–1) = 40 ± 2 and S (JK^–1 mol^–1) = – 61 ± 8. The results also indicated that the formation of NiAEB²⁺ involves a chelation-controlled, rate-limiting process. Analysis of the S ° data for the acid ionisation of AEBH _inf2 ^p2+ and the formation of NiAEB²⁺ showed that the bulky AEBH⁺ ion has a solvent structure breaking effect as compared to AEB [s _aqS ° (AEBH⁺) – s _aq ° (AEB) = 69 JK^–1 mol^–1], while AEBH _inf2 ^p2+ is a solvent ordering ion relative to NiAEB²⁺ [s _aq° (NiAEB²⁺) – ovS _aq ° (AEBH _inf2 ^p2+ ) = 11 JK^–1 mol^–1].Author to whom all correspondence should be directed.     

Kinetic and mechanistic studies on the interaction of<Emphasis Type="SmallCaps">DL</Emphasis> -Penicillamine with the hydroxopentaaquarhodium(III) ion

The kinetics of interaction between DL-Penicillamine and [Rh(H₂O)₅OH]²⁺ have been studied spectrophotometrically as a function of [Rh(H₂O)₅OH²⁺], [DL-Pen], pH and temperature. The reaction has been monitored at 242 nm, the _max of the substituted complex and where the spectral difference between the reactant and product is a maximum. The reaction rate increases with [DL-Pen] and reaches a limiting value at a higher ligand concentration. From the experimental findings an associative interchange mechanism for the substitution process is suggested. The activation parameters (H}=35.8 ± 1.6 kJ mol^–1, S=–209 ± 5 J K^–1 mol^–1) support the proposition. The negative G ⁰ (–13.6 kJ mol^–1) for the first equilibrium step also supports the spontaneous formation of an outersphere association complex.     

Kinetic and mechanistic studies on the interaction of thymidine with the hydroxopentaaquarhodium(III) ion

The interaction of thymidine, a nucleoside, with hydroxopentaaquarhodium(III), [Rh(H₂O)₅(OH)]²⁺ ion in aqueous medium is reported and the possible mode of binding is discussed. The kinetics of interaction between thymidine and [Rh(H₂O)₅OH]²⁺ has been studied spectrophotometrically as a function of [Rh(H₂O)₅OH²⁺], [thymidine], pH and temperature. The reaction has been monitored at 298 nm, the _max of the substituted complex, and where the spectral difference between the reactant and product is a maximum. The reaction rate increases with [thymidine] and reaches a limiting value at a higher ligand concentration. From the experimental findings an associative interchange mechanism for the substitution process is suggested. The activation parameters (H=47.8 ± 5.7 kJ mol^–1, S=–173 ± 17 J K^–1 mol^–1) supports our proposition. The negative G⁰ (–13.8 kJ mol^–1) for the first equilibrium step also supports the spontaneous formation of the outer sphere association complex.     

Kinetics and mechanism of the oxidation of acetaldehyde by chromic acid in perchloric acid

Summary The oxidation of MeCHO by chromium(VI) has been studied in HClO₄ medium over a wide range of experimental conditions and has been found to obey the rate law;v=k[MeCHO][HCrO ₄ ^– ][H⁺]. The calculated H and-S values for the reaction are 30±2kJ mol^–1 and 171±7J mol^–1deg^–1, respectively. The mechanism is discussed in terms of carbon-hydrogen bond cleavage.     

Mechanism of oxidation of selenium(IV) by cobalt(III) in perchloric acid — a kinetic study

Summary The kinetics of Co^III oxidation of Se^IV have been studied in aqueous HClO₄. The order with respect to Co^m is two the order with respect to Se^IV is one at low concentrations; two at high concentrations. The latter variation is attributed to the greater reactivity of the Se^IV dimier A mechanism involving complexation between oxidant and substrate is proposed. [CoOH]²⁺ is presumed to be the reactive Co^III species and H₂SeO₃ and HSeO ₃ ^– to be those of Se^IV. At 25° C, E_a, H and S for the monomeric path are 125.6±4.0 kJ mol^–1, 122.1±3.8 kJ mol^–1 and 206±12 JK^–1 mol^–1 respectively and those for the dimeric path are 88.6±3.6 kJ mol^–1, 85.9±3.4 kJ mol^–1 and 62.6±11.3 JK^–1 mol^–1 respectively.     

Kinetics and mechanism of the reactions ofcis-tetraaminediaqua-cobalt(III) with nitrite,azide and salicylate ions in aqueous acidic media

Summary Kinetic studies of the anation of the title complex by NO ₂ ^– show that it occurs in a stepwise manner leading to thecis-dinitro-complex both steps having a common rate equation:-d[complex]/dt = a[NO ₂ ^– ]/{[NO ₂ ^– ] + b}. The variation ofpseudo-first-order rate constant (k_obs) with [NO ₂ ^– ] indicates that the reaction proceeds through ion-pair interchange path. Activation parameters calculated by the Eyring equation are: H ₁ = (65±7) kJ mol^–1 and S ₁ = (–82±11) JK^–1 mol^–1 for the formation of [Co(NH₃)₄(NO₂)(H₂O)]²⁺, and H ₂ = (97±1) kJ mol^–1 and S ₂ = (6±2) JK^–1 mol^–1 for the formation of [Co(NH₃)₄(NO₂)₂]⁺. Anation of the title complex by N ₃ ^– at pH 4.1 also occurs in a stepwise manner ultimately producing thecis-diazido species. At a fixed pH the reaction shows a first-order dependence on [N ₃ ^– ] for each step. pH-variation studies at a fixed [N ₃ ^– ] show that the hydroxoaqua-form of the complex reactsca. 16 times faster than the diaqua form. Evidence is presented for an ion-pair preequilibrium at high ionic strength (I = 2.0 mol dm^–3). Activation parameters obtained from temperature variation studies are: H ₁ = (121±1) kJ mol^–1 and S ₁ = (104±3) JK^–1 mol^–1 (for the first step anation), and H ₂ = (111±2) kJ mol^–1 and S ₂ = (74±9) JK^–1 mol^–1 (for the second step anation). The reaction ofcis-tetraaminediaquacobalt(III) ion with salicylate (HSal^–) has been studied in aqueous acidic medium in the temperature range 39.8–58.2°C. The reaction is biphasic corresponding to the anation of two salicylate ions. The kinetic results for the first phase reaction are compatible with the equation: k_obs = k_IPQ[HSal^–]/(1 + Q[HSal^–]) where Q denotes ion-pair formation constant and k_IP is the first-order rate constant for the interchange reaction. The activation parameters obtained from the temperature dependence of rate are: H = (138±3) kJ mol^–1 and S = (135±4) JK^–1 mol^–1. The reaction seems to take place by a dissociative interchange mechanism.     

Superionic conductivity in (BEDT-TTF)AgXiY

We have synthesized the organic conductor (BEDT-TTF)Ag_XI_Y (X 1.8 and Y 2.9). This compound has, in addition to high electronic conductivity (_{300 k} 5–10 ^–1 · cm^–1), significant ionic conductivity connected with the motion of silver ions. The value of this ionic conductivity at room temperature is 10^{–3 –1} · cm^–1. The activation energy for diffusion of Ag ions is equal to 0.2 eV.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 247–249, March–April, 1989.