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.     

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.     

Spectrochemistry of solutions,part 23. Changes of enthalpy and entropy in the formation of contact ion pairs: A vibrational spectroscopic appraisal using thiocyanate and azide solutions

Summary The vibrational spectra of solutions have been analyzed to assess both qualitatively and quantitatively the changes in enthalpy and entropy for ion pair formation in solutions of LiNCS, Mg(NCS)₂, and LiN₃ in liquid ammonia, dimethylformamide, dimethylsulphoxide and acetonitrile. Contrary to predictions both the H _ass and S _ass terms are all positive in the cases examined, indicating that the driving force in the ion association process derives from solvent-solute restructuring, and not the energy of the interaction between the cation and anion. This characteristic of contact ion pair formation is likely to be found to be applicable over a wide range of solvents. The following specific values of the thermodynamic parameters at 298 K have been obtained: LiNCS/DMF, G=–1.3 (1) kJ mol^–1, H _ass =+1.8 (5) kJ mol^–, S _ass =+10 (2) J mol^–1 K^–1; LiNCS/DMSO, G=+0.9 (2) kJ mol^–1, H _ass =+0.3 (3) kJ mol^–1; Mg(NCS)₂/DMF, G _ass =–4.0 (3) kJ mol^–1, H _ass =+15 (4) kJ mol^–1, S=+64 (17) kJ mol^–1; LiN₃/DMSO, G _ass =–2.5 (3) kJ mol^–1, H _ass =+4.9 (9) kJ mol^–1, S _ass =+25 (10) J K^–1 mol^–1.Submitted to celebrate the 70th Birthday of Professor Viktor Gutmann, and in recognition of his considerable contributions towards the better understanding of Chemistry in the Solution Phase     

Kinetics and mechanism of the interaction of thiourea with cis-diaquaethylene-diamineplatinum(II) perchlorate in aqueous medium

The kinetics of the interaction of thiourea with [Pt(en)(H₂O)₂]²⁺ have been studied spectrophotometrically as a function of [Pt(en)(H₂O)₂]²⁺, [thiourea] and temperature at a particular pH(4.0), where the substrate complex exists predominantly as the diaqua species and the thiourea ligand as a neutral molecule. The reaction proceeds via a rapid outer sphere association followed by two slow consecutive steps, the second step exhibiting first order dependence on the aqua ion and thiourea concentrations. The activation parameters for both the steps have been evaluated: (H ₁ = 54.8 ± 1.2 kJ mol^–1, S ₁ = –96 ± 4 J K^–1 mol^–1, H ₂ = 27.9 ± 0.8 kJ mol^–1 and, S ₂ = –183 ± 2.6 J K^–1 mol^–1). The low enthalpy of activation and large negative values of entropy of activation indicate an associative mode of activation for both consecutive steps.     

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.     

A new route to the asymmetric Schiff base ligands. The ligand exchange in [Mo(CN)<Subscript>3</Subscript>O(salhy)]<Superscript>2−</Superscript> ion. Kinetics and mechanism

The kinetics of the ligand exchange in (PPh₄)₂[Mo(CN)₃O(salhy)]. 6H₂O (Hsalhy = salicylaldehyde hydrazone) by a solvent molecule and by 2,2-bipyridine (bpy) have been studied in EtOH. For the ligand exchange by a solvent molecule the pseudo-first order rate constant equals k _obs = 3.2 (±0.2) × 10^–3 s^–1 (t=25 °C), H =67 (± 7) kJ mol^–1, S =–75 (±23) J mol^–1 K^–1, while for the exchange by a bpy molecule k _obs=3.5 (±0.2) × 10^–3 s^–1 (t=25 °C), H =56 (±7) KJ mol^–1, S = –104 (±8) J mol^–1 K^–1. It was found, that all reactions proceed via the same mechanism which involves the chelate ring opening cis to the Mo=O bond. The mechanism of the reaction was proposed and was proved by the synthesis of (PPh₄)₂[Mo(CN)₃O(N-pic)]. 2.5H₂O (N-pic denotes that the nitrogen of picolinic acid is trans to Mo=O) by ligand exchange in EtOH, while in aqueous solution the O-pic analogue is formed exclusively.     

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.     

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.     

Stereochemical nonrigidity of (O—Ge)-chelate bis[(2-oxo-1-hexahydroazepinyl)methyl]dichlorogermane: substantiation of the dissociative mechanism of chelating ligand exchange

Stereochemical nonrigidity of the hexacoordinated (O—Ge)-chelate bis(2-oxo-1-hexahydroazepinylmethyl)dichlorogermane in CDCl₃ was studied by dynamic NMR. The activation parameters of the intramolecular rearrangement at the coordination center are G ^# ₂₉₈ = 12.3±0.2 kcal mol^–1, H ^# = 16.9±0.2 kcal mol^–1, and S ^# = 15.3±0.7 cal mol^–1 K^–1. The dissociative mechanism of ligand exchange involving the cleavage of the OGe coordination bond is discussed based on the positive entropy of activation.     

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.     

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.     

The influence of solvent structure on the solvolysis oftrans-dichlorotetra (4-ethylpyridine) cobalt(III) perchlorate

Summary The solvolysis oftrans-[Co(4-Etpy)₄Cl₂]ClO₄, was followed spectrophotometrically in water/isopropanol at different temperatures. The activation energy varied nonlinearly with the mole fraction of the co-solvent, ₂. The plot of logk versus D _s ^–1 was also non-linear. These features were attributed to the differential solvation of the initial and transition states. On plotting H versus S, the points fall very close to straight line. The isokinetic temperature was found to be 334K, indicating that the solvolysis reaction is controlled by S and not H. The change in H and S with the mole fraction of the cosolvent shows extrema at the composition range where changes in solvent structure occur. The influence of the solvent structure on the complex ion in the transition state dominates over that in the initial state, where –G _t ⁰ [Co(4-Etpy)₄Cl]²⁺>–G _t ⁰ [Co(4-Etpy)₄Cl₂]⁺.     

Thermodynamics of tri- and pentabromide anions in aqueous solution

Formation constants for the tribromide and pentabromide anions were measured by a vapor partitioning method from 5 to 80°C. The molal thermodynamic parameters for these respective species at 25°C are: K ₃ –16.73, H ^o =–5.90 kJ-mol ^–1 , Cp ^o =–29 J-K ^–1 -mol ^–1 , and S ^o =3.6 J-K ^–1 -mol ^–1 ; K ₅ =37.7, H ^o =–13.0 kJ-mol ^–1 , S ^o =–13.6 J-K ^–1 -mol ^–1 , with Cp ^o assumed zero. These results are used to reevaluate published emf results for the bromine/bromide couple.     

Formation Enthalpy and Entropy of Exciplexes with Variable Extent of Charge Transfer in Solvents of Different Polarity

Temperature dependence was studied for relative quantum yields of emission from some exciplexes of pyrene, 1,12-benzoperylene, and 9-cyanoanthracene with methoxybenzenes or methylnaphthalenes in solvents of different polarity (ranging from toluene to acetonitrile). The enthalpy H _Ex ^*, the entropy S _Ex ^*, and the Gibbs free energy G _Ex ^*of formation of the exciplexes were determined. Depending of the Gibbs free energy of excited-state electron transfer (G _et ^*) and solvent polarity, the values of H _Ex ^*, S _Ex ^*, and G _Ex ^*vary over the ranges from –5 to –40 kJ mol^–1, from +3 to –90 J mol^–1K^–1, and from +3 to –21 kJ mol^–1, respectively. The possibility is discussed that the effect of solvent polarity G _et ^*on the exciplex formation enthalpies can be rationalized in terms of the model of correlated polarization of an exciplex and the medium.     

Nuclear magnetic resonance study of the ligand interchange of Ba2+-18-crown-6 complex in methanol solution

Exchange kinetics of Ba²⁺-18-crown-6 complex in deuterated methanol solution was studied by proton NMR line-shape analysis of a series of solutions containing equal population of free and complexed 18-crown-6, but varying concentration of the macrocycle, at various temperatures. From –33 to 37°C, the predominant mechanism for the exchange of the ligand between the two sites is a bimolecular pathway which is characterized by the following activation parameters:E _a=47±2 kJ-mol^–1; H =45±2 kJ-mol^–1; S =–8±4 J-mol^–1-K^–1. However, the contribution of a dissociative mechanism with activation parametersE _a=36±5 kJ-mol^–1, H =33±5 kJ-mol^–1 and S =104±18 J-mol^–1-K^–1 becomes more important at higher temperatures.     

Kinetics and mechanisms of halide ion catalysis of CO substitution reactions in Os3(CO)12 and Ru3(CO)12 metal carbonyl clusters

The results of kinetic studies on ligand substitution in [M₃(CO)₁₁X]^– complexes (M = Ru, Os; X = Cl, Br, I) are summarized. The [Os₃(CO)₁₁X]^– complexes react with PPh₃ under mild conditions to initially yield monosubstituted products [Os₃(CO)₁₀(PPh₃)X]^–. The rate of CO substitution obeys a first-order equation with respect to the concentration of the complex and does not depend on the ligand concentration. The rates of the reactions decrease in the order Cl > Br > I withH values increasing from 15 to 18 kcal mol^–1 and S values varying from –19 to –13 cal mol^–1 K^–1. The enhanced reactivities of these complexes as well as the low activation energies and negative activation entropies are discussed in terms of the effects of -X bridge formation on the transition state of the reaction. Reactions of PPN[Ru₃(CO)_11–x (Cl)] (PPN is the bis(triphenylphosphine)iminium cation;x=0, 1) and PPN[Ru₃(CO)₉(₃-I)] with alkynes are also reported. The reactivities of alkynes follow the order BuCCH PhCCH EtCCEt PhCCPh. The higher rates of the reactions of monosubstituted acetylenes compared with those of their disubstituted analogs are explained by agostic interaction between the metal atom and the C-H bond in the reaction transition state and by steric effects. The results obtained attest that the reaction with alkynes occursvia intermediates containing halide bridges and that ₃-halide complexes are more reactive than ₂-halide complexes.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1540–1545, September, 1994.This work was supported by a Presidential Grant from Northwestern University. One of the authors (F. Basolo) wishes to thank Academician M. E. Vol'pin for the invitation to participate in the Workshop The Modern Problems of Organometallic Chemistry (INEOS-94) and Academician O. M. Nefedov for the invitation to publish a review in theRussian Chemical Bulletin.     

Far infrared spectra (500–50 cm−1) of the 2,2′-bipyridine complexes of palladium and platinum halides

Summary The far-i.r. spectra of the title complexes have been examined. Band assignments are based on the shifts induced by ligand deuteration and halide substitution. Deuteration of bipyridine causes large shifts ( >10 cm^–1) in internal ligand modes, intermediate shifts between 2 and 9 cm^–1) in metal-nitrogen stretching and bending modes and small to zero shifts in metal-halide stretching and bending vibrations. Generally, the requirements for square planarC _2v synanetry [two (M–N) and two (M–X) bands] are observed. Previous ambiguities in the assignment of the (M–N) bands have been resolved by the isotopic labelling technique employed in this study.     

Kinetics of the thermal and photochemical decomposition of aquapentacyanoferrate(III) in aqueous solution

Summary The kinetics of the thermal and photochemical decomposition of aquapentacyanoferrate(III) ion in aqueous solution in the presence ofo-phenanthroline was studied spectrophotometrically. The first-order rate constant (k ) at 30° C [I=1 M(NaCl)] for the thermal reaction is (1.49±0.13)×10^–6 s^–1 with H =(158±7)kJ mol^–1 and S=(42±4) JK^–1 mol^–1. The initial quantum yield for the photochemical reaction at pH=7 is independent of the light intensity and is (1.49±0.33)×10^–2 mol einstein^–1.A communication on this subject was presented at the XVI Latinamerican Chemistry Congress held at Rio de Janeiro. Brasil, October 14–20, 1984.     

Reactivity of tungsten(0) and molybdenum(0) pentacarbonyl thiobenzoate anions: thiobenzoate as a cis-CO labilizing ligand

The [Et₄N][M(CO)₅SCOPh] complexes (1a, M = Mo; 2a, M = W) have been prepared at ambient temperatures by reacting the photogenerated M(CO)₅ THF intermediate with [Et₄N][SCOPh] in THF. Kinetic studies of the reactions of the anions [M(CO)₅SCOPh]^– with the tri(iso-propyl)phosphite (L) ligand under pseudo-first-order conditions indicate that these reactions are first-order in substrate and are independent of the P(OPr-i)₃ concentration. It is thus envisaged that these CO substitutions proceed via a mechanism which involves initial cis-M—CO bond-breaking, followed by fast attack of the incoming nucleophile on the resulting intermediate to give [cis-M(CO)₄{P(O-Pri)₃}SCOPh]^–. This facile displacement of cis-CO indicates the labilizing nature of the thiobenzoate ligand, most probably by virtue of distal oxygen atom participation. Activation parameters for the reactions are: [M(CO)₅SCOPh]^– + L cis-[M(CO)₄(L)SCOPh]^– + CO M = Mo, H = 24.6(2) kcal mol^–1, S = 8.2(6) eu; M = W, H = 28.4(2) kcal mol^–1, S = 11.3(5) eu. Kinetic data and the mechanism of these ligand-substitutions are discussed.