KINETICS OF FORMATION AND STABILITY CONSTANTS OF MONO AND BIS l-TYROSINE COMPLEXES OF Cu(II), Co(II), AND Ni(II)

Abstract

The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)^(3-n)+ _n-1 + HL- ? M(HL)^(2-n)+_n(k_n, forward rate constant; k_-n, reverse rate constant); where M=Cu, Co or Ni, HL^? refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (K_n=k_n/k_-n) of the mono and bis complexes of Cu²⁺, Co²⁺ and Ni²⁺ with l-tyrosine, determined by potentiometric pH titration are: Cu²⁺, log K₁=7.90 ± 0.02, log K₂=7.27 ± 0.03; Co²⁺, log K₁=4.05 ± 0.02, log K₂=3.78 ± 0.04; Ni²⁺, log K₁=5.14 ± 0.02, log K₂=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu²⁺, k₁=(1.1 ± 0.1) × 10⁹ M ^?1 sec^?1, k_-1=(14 ± 3) sec^?1, k₂=(3.1 ± 0.6) × 10⁸ M ^?1 sec^?1, k^?2=(16 ± 4) sec^?1; Co²⁺, k₁=(1.3 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-1=(1.1 ± 0.2) × 10² sec^?1, k₂=(1.5 ± 0.2) × 10⁶ M ^?1 sec^?1, k_-2=(2.5 ± 0.6) × 10² sec^?1; Ni²⁺, k₁=(1.4 ± 0.2) × 10⁴ M ^?1 sec^?1, k_-1=(0.10 ± 0.02) sec^?1, k₂=(2.4 ± 0.3) × 10⁴ M ^?1 sec^?1, k_-2=(0.94 ± 0.17) sec^?1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus.     

Mixed Ligand Complexes of Cobalt(II) – Synthesis,Structure, and Properties of Co4(thd)4(OEt)4

Synthesis, crystal structure, thermal stability, and magnetic properties of mixed‐ligand complexes of cobalt(II) with ß‐diketonato (thd = C₁₁H₁₉O₂^?) and alkoxides (OR mainly OMe = methoxide = CH₃O^? or OEt = ethoxide = C₂H₅O^?) are reported. Direct reaction between Co(thd)₂ ( 1 ) and EtOH gives a new complex with the structural formula [Co₄(thd)₄(OEt)₄] ( 2 ) whereas MeOH correspondingly reacts to [Co₄(thd)₄(OMe)₄(MeOH)₄] ( 3 ). The yield of these products decreases with increasing size of the R group owing to increased solubility of 1 in the alcohol. The structure of 2 is determined from single‐crystal X‐ray diffraction data. At 100 K 2 takes a monoclinic structure (space group C2/c): a = 15.108(2), b = 19.428(2), c = 21.240(3) Å, and β = 108.882(2)°. At 295 K 2 has transformed to a closely related orthorhombic structure (space group Fddd): a = 15.233(3), b = 19.712(3), c = 40.916(7) Å. Protracted hydrolysis accompanied by oxygenation of complexes 2 and 3 in laboratory air (viz. simultaneous exposure to moisture and oxygen) leads to a new complex 4 with empirical formula corresponding to [Co(thd)(OH)(O₂)]. Magnetic susceptibility data show that Co takes the valence state II in all complexes 1 – 4 . For 4 this implies that dioxygen has to form an adduct‐like association to the rest of the complex. Unfortunately complex 4 has hitherto only been obtained in the amorphous state, but all here produced evidences point at 4 as a distinct entity and that products of 4 obtained from 2 and 3 are chemically identical (but differ somewhat with regard to short‐ and longer‐range order in the atomic arrangement). The interatomic distances in the crystal structure of complexes 1 – 3 are briefly discussed in terms of the bond‐valence concept.     

Characterization of Anilinepentacyanoferrate (II) and (III)

The anilinepentacyanoferrate (II) complex has been characterized in aqueous solution. The complex exhibits a predominant ligand field transition at λ_max = 415 nm with ?_max = 494 M^?1 cm^?1. The corresponding Fe(III) complex displays a strong absorption at λ_max = 700nm(?_max = 1.61×10⁴ M^?1 sec^?1) which can be assigned as a ligand to metal charge transfer transition. The rate constants of formation and dissociation for the Fc(II) complex are (3.14±0.18)×10² M^?1W^?1 and 0.985±0.005 sec^?1, respectively, at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The cyclic voltammetry of the complex shows that a reversible redox process is observed with E_1/2 value of 0.51±0.01 V vs. NHE at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C. The kinetic study of the oxidation of the Fe(II) complex by ferricyanide ion yielded the rate constant of the reaction k_et = (1.43±0.04)x10 M sec^?1 at μ = 0.10 M LiClO₄, pH = 8 and T = 25°C.     

The Kinetics of the Anation Reaction of Aquopentaamminerhodium(III) lon by Oxalate in Aqueous Acidic Solution

The anation reaction of aquopentaamminerhodium(III) by oxalate has been studied in the temperature range 51–69°C and acidity range 0 ≤ pH ≤ 4.5 for oxalate concentrations up to 0.25 M and at ionic strength 1.0 M. The kinetic results provide evidence for the formation of an ion-pair between the complex ion and HC₂O(Q₁) and C₂O₄^2?(Q₂), where Q₁ = 2.3 M^?1 and Q₂ = 8.1 M^?1 at 60°C, but no evidence for an ion-pair with H₂C₂O₄ exists. The values of the rate constants at 60°C for anation by H₂C₂O₄, HC₂O and C₂O₄^2? are k₀ = 1.5 · 10^?4 M^?1 sec^?1, k₁ = 1.4 · 10^?4 sec^?1 and k₂ = 1.2 · 10^?4 sec^?1. The corresponding values for ΔH≠ and ΔS≠ are reported and the results discussed with reference to analogous reactions of Rh(III) and Co(III).     

Synthesis,Characterization, and Reactivity of Cobalt(III)–Oxygen Complexes Bearing a Macrocyclic N‐Tetramethylated Cyclam Ligand

Mononuclear metal–dioxygen species are key intermediates that are frequently observed in the catalytic cycles of dioxygen activation by metalloenzymes and their biomimetic compounds. In this work, a side‐on cobalt(III)–peroxo complex bearing a macrocyclic N‐tetramethylated cyclam (TMC) ligand, [Co^III(15‐TMC)(O₂)]⁺, was synthesized and characterized with various spectroscopic methods. Upon protonation, this cobalt(III)–peroxo complex was cleanly converted into an end‐on cobalt(III)–hydroperoxo complex, [Co^III(15‐TMC)(OOH)]²⁺. The cobalt(III)–hydroperoxo complex was further converted to [Co^III(15‐TMC‐CH₂‐O)]²⁺ by hydroxylation of a methyl group of the 15‐TMC ligand. Kinetic studies and ¹⁸O‐labeling experiments proposed that the aliphatic hydroxylation occurred via a Co^IV–oxo (or Co^III–oxyl) species, which was formed by O? O bond homolysis of the cobalt(III)–hydroperoxo complex. In conclusion, we have shown the synthesis, structural and spectroscopic characterization, and reactivities of mononuclear cobalt complexes with peroxo, hydroperoxo, and oxo ligands.     

A kinetic study of the oxidation of L-ascorbic acid by chromium(VI)

The reaction between chromium(VI) and L-ascorbic acid has been studied by spectrophotometry in the presence of aqueous citrate buffers in the pH range 5.69–7.21. The reaction is slowed down by an increase of the ionic strength. At constant ionic strength, manganese(II) ion does not exert any appreciable inhibition effect on the reaction rate. The rate law found is where K_p is the equilibrium constant for protonation of chromate ion and k_r is the rate constant for the redox reaction between the active forms of the oxidant (hydrogenchromate ion) and the reductant (L-hydrogenascorbate ion). The activation parameters associated with rate constant k_r are E_a = 20.4 ± 0.9 kJ mol^?1, ΔH^≠ = 17.9 ± 0.9 kJ mol^?1, and ΔS^≠=?152 ± 3 J K^?1 mol^?1. The reaction thermodynamic magnitudes associated with equilibrium constant K_p are ΔH⁰ = 16.5 ± 1.1 kJ mol^?1 and ΔS⁰ = 167 ± 4 J K^?1 mol^?1. A mechanism in accordance with the experimental data is proposed for the reaction. © 1993 John Wiley & Sons, Inc.     

Substitution and Isomerization of Nitrite Ion at a Cobalt(III) Centre Having a Coordinated Pentadentate Ligand with Two Nitrogen and Three Sulfur Donor Atoms

The influence of placing thioether linkages trans to a site of nitrito substitution and spontaneous nitrito-tonitro isomerization is reported for the [CoQS(H₂O)]³⁺ cation where QS is 1,11-diamino-3,6,9-trithiaundecane. Preparation and characterization is described for the aqua and nitrito complexes. Rate data for the substitution process is presented at 17.7, 25.0 and 35.0°C. It is consistent with the mechanism first proposed by Basolo and Pearson in which N₂O₃ is the nitrosation agent. [CoQS(H₂O)]³⁺ is three hundred times more reactive than [Co(NH₃)₅H₂O]³⁺ under identical conditions. Isomerization is dramatically slower than the conversion of [CoQS(H₂O)³⁺ to [CoQS(ONO)]²⁺. The isomerization process was studied at 5 wavelengths, 3 temperatures and various conditions of acid and nitrite ion at an ionic strength of 0.11–0.60 M. Studies at 25°C give k_isom = 1.21 ± 0.12 × 10^?4 sec ^?1. Similar determinations at 17.7 and 35.0°C give k_isom = 3.84 ± 0.65 × 10^?5 sec^?1 and 3.59 ± 0.13 × 10^?4 sec^?1 respectively. The thermodynamic activation parameters ΔH^≠, ΔG^≠, and ΔS^≠ obtained from an Eyring plot gives ΔH^≠ = 111.3 kJ/mol, ΔS^≠ = + 53 J/molK and ΔG^≠ = 95.4 kJ/mol. These results are discussed in the context of present knowledge and experience with other cobalt(III) ligand systems.     

Contributions on Thermal Behaviour and Crystal Chemistry of Anhydrous Phosphates. XXXIV [1] Oxygen Equilibrium Pressures in Ternary Systems M / P / O (M = Co,Ni) and Heats of Formation of Anhydrous Cobalt(II) and Nickel(II) Phosphates

Oxygen equilibrium pressures have been measured in the temperature range 800 °C to 1000 °C by coulometric/potentiometric techniques for several equilibrium regions in the ternary systems M / P / O (M = Co, Ni). In both systems oxygen coexistence pressures of three‐phase equilibrium solids phosphide/phosphate are about 3 to 5 orders of magnitude smaller than p(O₂) above the corresponding M_s / MO_s system. Heats of formation Δ_fH°₂₉₈ and standard entropies S°₂₉₈ for the phosphates have been obtained from 2^nd and 3^rd law evaluation of the temperature dependence of the oxygen coexistence pressures. Thermodynamic data from literature for the phosphides of cobalt and nickel and estimated heat capacities for the anhydrous phosphates Co₃(PO₄)₂, Co₂P₂O₇, Ni₃(PO₄)₂, Ni₂P₂O₇ and Ni₂P₄O₁₂ were used for these calculations. Thus obtained enthalpies and entropies are compared to results from thermodynamic modelling of observed solid phase equilibria in the ternary systems M / P / O (M = Co, Ni).     

On the mechanism of interaction between copper (I) and O2

The mechanism of the interaction of Cu⁺-α,α-dipyridyl complex (Cu⁺L₂) with O₂ in both neutral and acid media was studied by the stopped-flow method. The dependence of the mechanism on the acidity of the medium was established. In an acid medium H⁺ participated in a direct O₂ reduction to HO₂ by interaction with an oxygen adduct L₂Cu⁺O₂ formed without displacement of ligand molecules. In a neutral medium the reaction rate was limited by inner sphere charge transfer from Cu⁺ to O₂ to form an oxygen “charge transfer” complex L₂CuO⁺₂. The latter interacted either with the second ion Cu⁺L₂ or with the free ligand, or else it dissociated, reversibly or irreversibly, to form a radical anion O^?₂. The bimolecular rate constants of the oxygen “adduct” and “charge transfer” complex formation appeared to be k_bi = (1.0 ± 0.1) × 10⁵ and (1.5 ± 0.2) × 10⁴M^?1?sec^?1, respectively. The effective termolecular rate constants of O₂ reduction to HO₂ in an acid medium (with contribution from H⁺) and to O^?₂ in a neutral medium (with contribution from α,α-dipyridyl) were k_ter = 2.7 × 10⁸ and 10⁷M^?2?sec^?1. The rate constants of the elementary steps were estimated. The auto-oxidation mechanism of the aquoion and complexes of Cu⁺ is discussed in terms of the results obtained.     

Kinetic Studies of the Catalytic Effect of Cobalt(II) Ions on the Substitution Reactions of the Ethylenediaminetetraacetatocobalt(III) Complex

The rate for the substitution reaction of Co(edta)^? with ethylenediamine was greatly enhanced by the presence of an excess of Co(II) ion in solution. The rate constant is (13±2) M^?-sec^?1 at μi=0.10M LiClO₄, pH=11.1, [en]=0.10M and T=25°C. The mechanism for the reaction is discussed on the basis of the Marcus theory for outer-sphere processes corrected for electrostatic effects. This catalytic effect was not observed when the Co(II) was present in small amount due to the stability of the Co(edta)^?2 complex toward substitution. The rate constant for direct substitution of Co(edta)^? under the same conditions has also been measured and the value is (3.66±0.40)×10^?4sec^?.     

Dynamics of a conformational change in aqueous 18-crown-6 by an ultrasonic absorption method

A temperature dependence study of the ultrasonic amplitudes, velocities, and relaxation times for a presumed conformational transition of noncomplexed aqueous 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) is discussed. At all temperatures a single relaxation was observed within a 15–255-MHz frequency range. The equilibrium constant for the presumed conformational transition \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CR}_1 \mathop \rightleftarrows\limits^{K_{12} } {\rm CR}_2 $\end{document} was determined to be K₂₁ = (2 ± 2) × 10^?2. The activation parameters are ΔH₂₁^≠ = 10.2 ± 1.0 kcal/mol, ΔS₂₁^≠ = 7.7 ± 0.2 cal/(mol·deg), ΔH₁₂^≠ = 7.4 ± 1.0 kcal/mol, and ΔS₁₂^≠ = 7.7 ± 0.2 cal/(mol·deg), while the thermodynamic enthalpy and entropy were found to be ?2.6 ± 1.0 kcal/mol and 0 ± 0.2 cal/(mol·deg), respectively. The rate constants at 25.0°C for the presumed conformational transition are k₂₁ = (1.0 ± 0.3) × 10⁷ sec^?1 and k₁₂ = (6.2 ± 0.2) × 10⁸ sec^?1.     

Reaction of tetramethyl-1,2-dioxetane with triphenylphosphine: Activation parameters for the formation of 2,2-dihydro-4,4,5,5-tetramethyl-2,2,2-triphenyl-1,3,2-dioxaphospholane

The reaction of tetramethyl-1,2-dioxetane ( 1 ) and triphenylphosphine ( 2 ) in benzene-d₆ produced 2,2-dihydro-4,4,5,5-tetramethyl-2,2,2-triphenyl-1,3,2-dioxaphospholane ( 3 ) in ?90% yield over the temperature range of 6–60°. Pinacolone and triphenylphosphine oxide ( 4 ) were the major side products [additionally acetone (from thermolysis of 1 ) and tetramethyloxirane ( 5 ) were noted at the higher temperatures]. Thermal decomposition of 3 produced only 4 and 5 . Kinetic studies were carried out by the chemiluminescence method. The rate of phosphorane was found to be first order with respect to each reagent. The activation parameters for the reaction of 1 and 2 were: Ea ? 9.8 ± 0.6 kcal/mole; ΔS^≠ = ?28 eu; k_30° = 1.8 m^?1sec^?1 (range = 10–60°). Preliminary results for the reaction of 1 and tris (p-chlorophenyl)phosphine were: Ea ? 11 kcal/mole, ΔS^≠ = ?24 eu, k_30° = 1.3 M^?1sec^?1 while those for the reaction of 1 and tris(p-anisyl)phosphine were: Ea ? 8.6 kcal/mole, ΔS^≠ = ?29 eu, k_30° = 4.9 M^?1 sec^?1.     

The very-low-pressure pyrolysis (VLPP) of n-propyl nitrate,tert-butyl nitrite,and methyl nitrite. Rate constants for some alkoxy radical reactions

The pyrolysis of n-propyl nitrate and tert-butyl nitrite at very low pressures (VLPP technique) is reported. For the reaction the high-pressure rate expression at 300°K, log k₁ (sec^?1) = 16.5 ? 40.0 kcal/mole/2.3 RT, is derived. The reaction was studied and the high-pressure parameters at 300°K are log k₂(sec^?1) = 15.8 ? 39.3 kcal/mole/2.3 RT. From ΔS_1,?1⁰ and ΔS_2,?2⁰ and the assumption E_?1 and E_?2 ? 0, we derive log k_?1(M^?1·sec^?1) (300°K) = 9.5 and log k_?2 (M^?1·sec^?1) (300°K) = 9.8. In contrast, the pyrolysis of methyl nitrite and methyl d₃ nitrite afford NO and HNO and DNO, respectively, in what appears to be a heterogeneous process. The values of k_?1 and k_?2 in conjunction with independent measurements imply a value at 300°K for of 3.5 × 10⁵ M^?1·sec^?1, which is two orders of magnitude greater than currently accepted values. In the high-pressure static pyrolysis of dimethyl peroxide in the presence of NO₂, the yield of methyl nitrate indicates that the combination of methoxy radicals with NO₂ is in the high-pressure limit at atmospheric pressure.     

Kinetics and mechanisms of oxidation by metal ions,part XII [1] oxidation of formaldehyde by alkaline osmium(VIII)

The stopped-flow measurements on the disappearance of alkaline osmium(VIII) at 402 nm indicated a first-order dependence each in [Os(VIII)] and [HCHO]. The pseudo first-order rate constant k_obs ([HCHO] ? [Os(VIII)]) decreased with increasing [OH^?]. The ionic strength, however, had no effect on k_obs. The rapid scan spectra of the reaction mixture indicated the formation of an inert complex which absorbs at 319 nm. Therefore the rate determining step is considered to involve the bimolecular collision between OsO₄(OH) and hydrated formaldehyde. The values of the rate limiting constant k and the equilibrium constant K_ha for the formation of the alkoxide ion from the reaction of hydrated formaldehyde with OH^? are evaluated. The equilibrium constant K_ha, within the experimental limits, is independent of temperature. The pK_a value, calculated from K_ha, is 13.62 ± 0.05 which is in good agreement with the pK_a value 13.27 for formaldehyde. The activation parameters, ΔH? = 40 ± 2 kJ mol^?1 and ΔS? = ?51 ± 6 JK^?1 mol^?1, for the rate limiting constant k are determined.     

Mechanistic Aspects of Ligand Substitution on the Hydroxopentaaquarhodium(III) Ion in Aqueous Solution by Sulfur‐Containing Bioactive Ligands

The kinetics of the interactions between three sulfur‐containing ligands, thioglycolic acid, 2‐thiouracil, glutathione, and the title complex, have been studied spectrophotometrically in aqueous medium as a function of the concentrations of the ligands, temperature, and pH at constant ionic strength. The reactions follow a two‐step process in which the first step is ligand‐dependent and the second step is ligand‐independent chelation. Rate constants (k₁ ～10^?3 s^?1 and k₂ ～10^?5 s^?1) and activation parameters (for thioglycolic acid: ΔH₁^≠ = 22.4 ± 3.0 kJ mol^?1, ΔS₁^≠ = ?220 ± 11 J K^?1 mol^?1, ΔH₂^≠ = 38.5 ± 1.3 kJ mol^?1, ΔS₂^≠ = ?204 ± 4 J K^?1 mol^?1; for 2‐thiouracil: ΔH₁^≠ = 42.2 ± 2.0 kJ mol^?1, ΔS₁^≠ = ?169 ± 6 J K^?1 mol^?1, ΔH₂^≠ = 66.1 ± 0.5 kJ mol^?1, ΔS₂^≠ = ?124 ± 2 J K^?1 mol^?1; for glutathione: ΔH₁^≠ = 47.2 ± 1.7 kJ mol^?1, ΔS₁^≠ = ?155 ± 5 J K^?1mol^?1, ΔH₂^≠ = 73.5 ± 1.1 kJ mol^?1, ΔS₂^≠ = ?105 ± 3 J K^?1 mol^?1) were calculated. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. The products of the reactions have been characterized from IR and ESI mass spectroscopic analysis. A rate law involving the outer sphere association complex formation has been established as      

Studies on the rate of isotopic oxygen exchange between [Re(py)4O2]+ and solvent water

The rate of oxygen exchange between trans-[Re(py)₄O₂]⁺ and solvent water in pypyH⁺ buffer solution follows simple first-order kinetics and both oxygens are equivalent. The half-life for isotopic oxygen exchange is about 12 h at a pH of 5.0, 25°C, and [py] = 0.10 M. The observed rate constant for exchange increases with acidity, in the pH range 4 to 6, decreases with [py], and is nearly independent of ionic strength. A small but significant increase of k_obs occurs with increasing complex concentration. The rate of exchange follows the rate equation k_obs/2 = k₀ + k₁/[py] with k₀ = 1.4 × 10^?5(2) s^?1 and k₁ = 4.7 × 10^?7(1) M, s^?1 at 25°C. The activation parameters for the reaction at pH = 7.15 (predominately the k₀ term) are: ΔH* = +137.(1) kJ/M and ΔS* = +126.(1) J/MK. The pH effect and complex concentration effect are discussed in mechanistic terms. These results are compared to those found for [Re(en)₂O₂]⁺ and [Re(CN)₄O₂]^3?.     

Homogeneous decomposition of vinyl ethers. The heat of formation of ethanal-2-yl

The thermal unimolecular decomposition of three vinylethers has been studied in a VLPP apparatus. The high-pressure rate constant for the retro-ene reaction of ethylvinylether was fit by log k (sec^?1) = (11.47 + 0.25) - (43.4 ± 1.0)/2.303 RT at <T> = 900 K and that of t - butylvinylether by log k (sec^?1) = (12.00 ± 0.27) - (38.4 ± 1.0)/2.303 RT at <T> = 800 K. No evidence for the competition of the higher energy homolytic bond-fission process could be obtained from the experimental data. The rate constant compatible with the C? O bond scission reaction in the case of benzylvinylether was log k (sec^?1) = (16.63 ± 0.30) - (53.74 ± 1.0)/2.303 RT at <T> = 750 K. Together with ΔH_f,300⁰(benzyl·) = 47.0 kcal/mol, the activation energy for this reaction results in ΔH_f,300⁰(CH₂CHO) = +3.0 ± 2.0 kcal/mol and a corresponding resonance stabilization energy of 3.2 ± 2.0 kcal/mol for 2-ethanalyl radical.     

Hydrolysis of plutonium(VI) at variable temperatures (283-343 K)

The hydrolysis of Pu^VI was studied at variable temperatures (283–343 K) by potentiometry, microcalorimetry, and spectrophotometry. Three hydrolysis reactions, mPuO₂²⁺+nH₂O=(PuO₂)_m(OH)_n+nH⁺), in which (n,m)=(1,1), (2,2), and (5,3), were invoked to describe the potentiometric and calorimetric data. The equilibrium constants (*β_n,m) were determined by potentiometry at 283, 298, 313, 328, and 343 K. As the temperature was increased from 283 to 343 K, *β_1,1, *β_2,2, and *β_5,3, increased by 1, 1.5, and 4 orders of magnitude, respectively. The enhancement of hydrolysis at elevated temperatures is mainly due to the significant increase of the degree of ionization of water as the temperature increases. Measurements by microcalorimetry indicate that the three hydrolysis reactions are all endothermic at 298.15 K, with enthalpies of (35.0±3.4) kJ mol^?1, (65.4±1.0) kJ mol^?1, and (127.7±1.7) kJ mol^?1 for ΔH_1,1, ΔH_2,2, and ΔH_5,3, respectively. The hydrolysis constants at infinite dilution have been obtained with the Specific Ion Interaction approach. The applicability of three approaches for estimating the equilibrium constants at different temperatures, including the constant enthalpy approach, the DQUANT equation, and the Ryzhenko–Bryzgalin model, were evaluated with the data from this work.     

The tris(picolinate)vanadate(II) ion: Redox properties and electron transfer kinetics with cobalt(III) amines

Vanadium(II) ions form with the pyridine-2-carboxylate ligand a deep blue, tris-substituted complex absorbing at 660 nm (ε = 7.2 × 10³ M^?1) cm^?1) with a shoulder at 450 nm. Reversible spectroelectrochemistry and cyclic voltammetry were observed for this complex, with E_{$\text{1}\text{2}$} = ?0.448 V vs NHE, and ΔS_rc^θ = ?6 cal · mol^?1 · deg^?1. Electron transfer kinetics with [CO(en)₃]³⁺ led to k₁₂ = 3100 M^?1 s^?, ΔH^≠ = 12.4 kcal · mol^?1 and ΔS^≠ = ?0.9 cal · mol^?1 · deg^?1 (I = 0.10 M). For the related [Co(NH₃)₆]³⁺ complex, k₁₃ = 1.9 × 10⁴ M^?1 s^?1. The self-exchange rate constant and activation parameters were analysed in terms of relative Marcus theory.     

The kinetics of the anation reaction of aquopentaamminecobalt(III) by malonate in acidic aqueous solution

The Co(NH₃)₅OH₂³⁺ ion reacts with malonate to form Co(NH₃)₅O₂CCH₂CO₂H²⁺ or Co(NH₃)₅O₂CCH₂CO₂⁺, depending on the pH of the reaction solution. The kinetics of this anation reaction have been studied as a function of [H⁺] for the acidity range 1.5 ≤ pH ≤ 6.0 in the temperature range of 60 to 80°C, the [total malonate] ≤ 0.5 M, and the ionic strength 1.0M. The anation by malonic acid follows second-order kinetics, the rate constant being 8.0 × 10^?5 M^?1·sec^?1 at 70°C, and the anations by bimalonate (Q₁, k₁) and malonate ion (Q₂, k₂) are consistent with an I_d mechanism. Typical values at 70°C for the ion pair formation constants are Q₁ = 1.3, Q₂ = 5.4M^?1; and for the interchange rate constants k₁ = 5.3 × 10^?4; k₂ = 7.3 × 10^?4 sec^?1. The activation parameters for the various rate constants are reported and the results discussed with reference to previously reported data for similar systems.