Kinetics and mechanism of Pb2+ ion promoted aquation of tris-oxalatochromate(III) in acid perchlorate media

Summary In low acid (0.02 M HClO₄) media, Pb²⁺ ion strongly catalyses the aquation of Cr(ox) ₃ ^3– to givecis-Cr(ox)₂(OH₂) ₂ ^– ion. The catalytic efficiency of Pb²⁺ as represented by the second order rate constant, k_pb (3.76 × 10^–4 M^–1 s^–1 at 25 °C; I, 1.0 M perchlorate), for the Pb²⁺ catalysed path is remarkably higher than might be expected on the basis of K_pb-ox, the first formation constant for the lead-oxalate complex. This catalytic superiority of Pb²⁺ has been found to result mainly from a comparatively much lower H (65.2 ±0.8 kJ mol^–1) value which more than compensates for the relatively unfavourable S value (–93.2 ±2.4 JK^–1mol^–1) for this catalysed path. This low S value is, however, in line with the entropy of hydration of Pb²⁺ ion. These facts, together with the different LFER plots, have been utilised to propose a plausible mechanism for such catalysed reactions.     

Kinetics of aquation of chloropentamminecobalt(III) perchlorate in ethylene glycol-water media

Summary The aquation of chloropentamminecobalt(III) ion has been studied in ethylene glycol-water mixtures up to 80% by weight at five temperatures from 35 to 55°C. Specific solvent effects on the reaction rate and mechanism have been investigated and discussed. The validity of our previously proposed equations, correlating the specific reaction rate with the dielectric constant of the medium has been examined. The thermodynamic parameters of activation have been calculated and presented as evidence for the solvation effects and in support of the proposed reaction mechanism.     

Kinetics of aquation of the trans-dichlorobis (N-methylethylenediamine) cobalt(III) ion in a water-methanol media

Summary The trans-[Co(meen)₂Cl₂]Cl complex was prepared and characterized by elemental analysis, and u.v.-vis. and i.r. spectroscopies. The kinetics of the primary aquation of trans-[Co(meen)₂Cl₂]⁺ in H₂O and H₂O-MeOH have been investigated over a wide range of solvent compositions and temperatures (45–60 °C). Plots of rate constants (log k) versus the reciprocal of the dielectric constant of the medium (D _infs ^sup−1 ) and Grunwald-Winstein values of the solvent (Y) were non-linear. The variation of enthalpies (ΔH ^*) and entropies (ΔS ^*) of activation with solvent composition have been determined. Plots of ΔH ^* or ΔS ^* versus the mole fraction of the solvent exhibit a maximum at x ₂ ca. 0.1 and a minimum of x ₂ ca. 0.3; a linear plot of ΔH ^* versus ΔS ^* is obtained. Furthermore, the cycle relating the free energy of activation in H₂O to that in H₂O-MeOH shows that changes in the solvent structure in H₂O-MeOH mixtures generally stabilize the five-coordinate cation in the transition state, more than the cation in the initial state as the mole fraction of MeOH increases. The results are discussed and compared with other related systems.     

Kinetics and mechanism of formation of penta-ammineglycinecobalt(III) ion from aquopenta-amminecobalt(III) ion and glycine in acid media

The rates of formation of penta-ammineglycinecobalt(III) ion from aquopenta-amminecobalt(III) ion and glycine in acidic media have been studied spectrophotometrically at different glycine concentration and different pH in the range of 50–70°C. The ΔH≠ and ΔSz≠ values are 27.6 kcal mole^?1 and +5.2 e. u. respectively, and increase in ionic strength causes only a slight acceleration of the rate. The results are consistent with a mechanism involving outer-sphere association between the aquopenta-amminecobalt(III) complex and glycine, followed by its transformation into the product by an essentially dissociative process in which rupture of the Co(III)? OH₂ bond is primarily important in the transition state (S_N1IP mechanism).     

Kinetics and mechanism of dissociation of oxalatobis(phenanthroline)cobalt(III) ion in aqueous hydrochloric acid media

The kinetics of dissociation of oxalatobis(phenanthroline)cobalt(III) ion into cis-diaquobis(phenanthroline)cobalt(III) ion in aqueous HClKCl media have been studied. The rate is first-order with respect to acid concentration (1–2 M) with a specific rate constant, k_H = 6.2 × 10⁻⁴M⁻¹min⁻¹ at 75°C (μ, 2 M); ΔH^≠ and ΔS^≠ values are 22.1 kcal mole⁻¹ and −18.7 e.u., respectively. The magnitude of the ΔS^≠ value appears consistent with a dissociation mechanism involving reaction of the conjugate acid form of the complex (S_N 1CA mechanism).     

Kinetics and mechanism of the aquation of pentaaqua (trichloroacetato-O)-chromium(2+) ion

The kinetics of the aquation of (H₂O)₅Cr(O₂CCCl₃)²⁺ have been examined at 35–55°C and 1.00M ionic strength with [H⁺] = 0.01?1.00M. The reaction follows the rate equation -d ln [Cr_total]/dt = (a[H⁺]^?1 + b + c[H⁺])/(1 + d[H⁺]), where [Cr_total] is the stoichiometric concentration of the complex. At 45°C a = (1.41 ± 0.03) × 10^?7M/s, b = (1.66 ± 0.02) × 10^?5 s^?1, c = (7.0 ± 0.8) × 10^?5M^?1·S^?1 and d = 2.3 ± 0.3M^?1. Two mechanisms consistent with this rate law are discussed, with evidence being presented in favor of an ester hydrolysis mechanism involving steady-state intermediates. Equilibrium and activation parameters were determined.     

Kinetics and mechanism of tris-quinolinatochromium(III) aquation in HClO<Subscript>4</Subscript> media

The chromium(III)-quinolinato complexes, [Cr(quinH)₃]⁰, [Cr(quinH)₂(H₂O)₂]⁺ and [Cr(quinH)(H₂O)₄]²⁺ (where quinH = N,O-bonded quinolinic acid anion), were obtained and characterized in solution. The tris-quinolinato complex undergoes acid-catalyzed aquation to give the diaqua-product, whereas subsequent ligand liberation processes are exceptionally slow. Kinetics of the aquation were studied spectrophotometrically over the 0.1–1.0 M HClO₄ range, at I = 1.0 M. The first aquation stage, the chelate-ring opening at the Cr-N bond, is much faster than the second one. The following rate laws were established: k _obs = k ₁ + k ₋₁/Q ₁[H⁺] and k _obs = k ₂ Q ₂[H⁺]/(1 + Q ₂[H⁺]), where k ₁ and k ₂ are the rate constants for the chelate-ring opening and the ligand liberation, respectively, k ₋₁ is the rate constant of the chelate-ring closure, Q ₁ and Q ₂ are the protonation constants of the pyridine nitrogen and 3-carboxylate group in the one-end bonded intermediate, respectively. Kinetic parameters have been determined and the mechanism has been discussed.     

Kinetics and mechanism of the oxidation of oxalic acid and bioxalate ion by [ethylene bis (biguanide)] silver (III) cation in aqueous perchlorate media

In acidic aqueous media the complex [Ag^IIILL⁻)³⁺[L = ethylenebis (bi-guanide)] reacts quantitatively with oxalic acid and bioxalate ion to produce CO₂, Ag⁺ and L. Kinetic evidence indicates preequilibrium adduct formation, but the reactions appear to be outer-sphere with these adducts (I₁ and I₂) acting as kinetic dead ends.No Ag⁺ catalysis could be detected.     

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.     

Kinetics of aquation ofcis- andtrans-chloro-nitrobis-(ethylene-diamine)cobalt(III) ion in binary aqueous mixtures

Kinetics of aquation ofcis- andtrans-[Co(en)₂NO₂Cl]⁺ were investigated in aqueous mixtures of MeOH, EtOH,i-PrOH andt-BuOH at 298.1 K. The values of transfer functions corresponding to the transfer of reactant and activated complex from H₂O to the solvent mixtures were calculated from kinetic measurements and from solubilities of the complex salt. The results of the analysis of solvent effect are discussed in terms of D and I_d mechanisms. TMC 2676     

Solvent effects on the aquation of chloropentammine-cobalt(III) perchlorate in ethanol-water

Summary The aquation kinetics of the chloropenta-aminecobalt(III) ion in H₂O–EtOH mixtures have been determined. A new correlation is described for calculating the chemical potential from kinetic data and molar thermodynamic excess functions for binary mixtures. The rate constants correlate well with Grunwald-Winstein solvating power Y parameter and with the dielectric constant of the medium. The data supports the D mechanism.     

Kinetics of aquation of pentaammine(substituted salicylato)cobalt(III) complexes by ferric ion

Summary The aquation of pentaammine (substituted salicylato) cobalt(III) complexes [(NH₃)₅CoO₂CC₆H₃(X)OH]²⁺,X = 5-SO₃, 5-Br, 5-NO₂, and 3-NO₂ in the presence of ferric ion was studied spectrophotometrically in the 65°–80° range. Ferric ion catalyses the aquation of the substratesvia formation of a reactive binuclear species.Reprints of this article are not available.     

Kinetics and mechanism of permanganate oxidation of pectin polysaccharide in acid perchlorate media

The kinetics of oxidation of pectin polysaccharide as a natural polymer by permanganate ion in aqueous perchloric acid at a constant ionic strength of 2.0 mol dm⁻³ has been investigated spectrophotometrically. The reaction time curves showed two distinct stages, the initial stage was relatively slow, followed by an increase in the rate of oxidation at longer times. The results of the initial rates reveal first-order kinetics in permanganate ion and fractional-order with respect to pectin concentration. Kinetic evidence for the formation of an intermediate complex between the polysaccharide and the oxidant is presented. The results obtained at various hydrogen ion concentrations showed that the reaction is acid catalyzed. The added salts lead to the prediction that Mn⁴⁺ and/or Mn³⁺ play an important role in the autoaccleration period kinetics. A tentative reaction mechanism consistent with the kinetic results is discussed.     

Kinetics and mechanisms of the reactions of unsymmetrical chelate complexes. Acid-catalyzed aquation of glycinatobis-(biguanide)cobalt(III) perchlorate

The presence of the unsymmetrical glycinato chelate in this complex, makes the positions of the two biguanides non-equivalent. Thus the acid-catalyzed dissociation of one or other of them, gives two different isomeric (α- and β-) diaquo- products. Up to ∼50°C, the dissociation of only one biguanide takes place with the formation of the α- diaquo product. The α- diaquo form isomerizes to the β- form at higher temperature. Direct formation of the β- form does not take place at any stage of the reaction. Aquation of the bis(biguanide) complex follows a dissociative pathway up to ∼50°C, above which it becomes a complicated one. Isomerization of the α- product takes place intramolecularly by two paths, one dependent and the other independent of acid concentration. Both the isomerization paths involve “bond-rupture” processes.     

Kinetics and mechanism of the reaction of aquachromium(III) with l-ascorbic acid in aqueous media

The kinetics of the reaction of l-ascorbic acid (H2A) with aquachromium(III) has been studied over the range 0.04 [H2A]T 0.16 mol dm–3, 3.5 pH 6.0, 25.0 t 50.0C, 0.03 I 1.0 mol dm–3 (KNO3) and 0% [MeOH] 20% (v/v). The reaction takes place via an outer-sphere association between Cr3+/CrOH2+ and H2A, followed by transformation of the outer- into an inner-sphere complex by slow interchange. The anation rate constants for Cr3+ and CrOH2+ species (k1 and k2) at 25C and I=1.0moldm–3 (KNO3) are 1.68×10–4 and 2.14×10–4s–1, respectively. H and S for the k1 and k2 paths are 27.0±2kJmol–1, –227±5J K–1mol–1 and 57.2± 4kJmol–1, –123±14JK–1mol–1, respectively. Anation of Cr(H2O)3+6 and Cr(H2O)5OH2+ follow an Ia path.     

Kinetics and mechanism of oxidation of quinols by tris(1,10-phenanthroline) iron(III) complexes in aqueous acidic perchlorate media

The kinetics of oxidation of several substituted quinols by a series of Tris(1,10-phenanthroline)iron(III) complexes has been investigated with a stopped-flow technique at 6.0 and 20.0°C. The reactions were found to be first order on both reactants and independent of acidity. The second-order specific rate constants were strongly dependent on free energy of reaction. An interpretation of the mechanism in the light of Marcus theory has been developed. The first electron abstraction with semiquinone radical formation has been suggested as the rate-determining step, and on this basis, intrinsic parameters of the reactions have been derived. A good agreement was found between experimental and computed data.