Gehan M. El-Subruiti

Solvent effects on the kinetics of solvolysis of trans-dichlorobis (N-methylethylenediamine)cobalt(III) ion in water–propan-2-ol and water–acetonitrile mixtures

First order solvolysis rates of the trans-dichlorobis (N-methylethylenediamine) cobalt(III) ion have been measured over a wide range of solvent compositions and temperatures in water–propan-2-ol and water–acetonitrile mixtures. The rate of solvolysis is faster in the former mixtures rather than the latter. Plots of log(rate constant) versus the reciprocal of the dielectric constant of the co-solvent, and also versus the Grunwald–Winstein Y-values are non-linear for both co-solvents; this non-linearity is derived from a large differential effect of solvent structure between the initial and transition states. However, extrema in the variation of enthalpy ΔH‡ and entropy ΔS‡ of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of ΔH‡ versus ΔS‡ were obtained and the isokinetic temperature indicates that the reaction is entropy controlled. The application of a free-energy cycle shows that changes in solvent structure affect the pentacoordinated cobalt(III) ion in the transition state more than the hexacoordinated cobalt(III) ion in the initial state. In addition, the stabilizing influence of changes in solvent structure is greater in propan-2-ol–water mixtures than in acetonitrile–water mixtures, and the difference becomes greater as the mole fraction, x2 of the organic co-solvent increases.

Kinetics and Thermodynamics of Solvolysis of a Cationic and an Anionic Chlorocobalt (III) Complex in Water–Ethanol Mixtures

Rate constants and derived thermodynamic activation parameters are reported for solvolysis of trans-[Co(3Mepy)4Cl2]+ and [Co(CN)5Cl]3− ions in water-rich mixtures of water with ethanol at various temperatures and are analyzed by initial- and transition-state contributions. The variation of enthalpies and entropies of activation with solvent composition show extrema in composition ranges where the physical properties of the mixtures, influenced by changes in solvent structures, also show extrema. From the application of a free-energy cycle to the process of the initial state going to the transition state, it is concluded for the solvolysis of both complexes that the Co(III) species in the transition state is more stable in water + ethanol mixtures than in the initial state.

Kinetics of solvolysis of the trans-dichlorobis(R-ethylenediamine)cobalt(III) ion (R = N-methyl or N,N′-dimethyl) in urea–water mixtures

First order solvolysis rates of the trans-dichlorobis(R-ethylenediamine)cobalt(III) ion (R = N-Me or N,N′-Me2) complex have been investigated in mixtures formed by adding urea to H2O, which enhances the dielectric constant and decreases solvent structure. Differential effects of the changes in solvent structure on the initial and transition states are found to be important factors controlling changes in the rate constant with solvent composition. The variation of the enthalpy and the entropy of activation with solvent composition are contrasted with their variations found for the solvolysis of the complexes in mixtures where solvent structure is enhanced by additions of a co-solvent to water. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the cationic cobalt(III) complexes in the transition state are more stable than the cationic cobalt complexes the initial state in the water + urea mixtures.

Solvent effects on the initial and transition states for the solvolysis of trans-dichlorobis-(N-methylethylenediamine) cobalt(III) complex in water–dimethyl sulfoxide mixtures

Solvent effects on the initial and transition states for the solvolysis of the trans-dichlorobis-(N-methylethylenediamine)cobalt(III), (meen), complex have been investigated in the 25–55 °C range in aqueous DMSO mixtures, of varying solvent composition up to 60% by vol. The log of the first order rate constant, k, varies non-linearly with the reciprocal of the dielectric constant at the same temperature, due to differential solvation of the initial and transition states. The changes in the enthalpy, ΔH‡, and entropy, ΔS‡, of activation with the mole fraction of the co-solvent show extrema at the composition range where the change in solvent structure occurs. The application of a free energy cycle to the process of the initial state going to the transition state suggests that the effect of solvent structure on the complex ion in the transition state dominates the initial state and that this effect increases as the mole fraction of co-solvent increases.

Kinetics of aquation of trans-dichlorobis(N,N′-dimethylethylenediamine)cobalt(III) ion in water–methanol in water–propan-2-ol media

The complex trans-[Co(dmen)2Cl2]Cl (dmen=N,N′-dimethylethylenediamine) has been prepared and characterized by elemental analysis, u.v.-vis. and i.r. spectra. The kinetics of the primary aquation of trans-[Co(dmen)2Cl2] in H2O, H2O–MeOH and H2O–i-PrOH have been examined over a wide range of solvent compositions and temperatures (40–55°C). Plots of rate constants (log k) versus the reciprocal of the dielectric constant of the medium (Ds−1) and Grunwald–Winstein values of the solvent (Y) were found to be non-linear. The variation of enthalpies (ΔH‡) and entropies (ΔS‡) of activation with solvent composition has been determined. Plots of ΔH‡ or ΔS‡ versus the mole fraction of each solvent exhibit extrema at x2=ca. 0.16 and 0.27 for MeOH and at x2=ca. 0.03 and 0.14 for i-PrOH. Furthermore, the cycle relating the free energy of activation in H2O to that in H2O–co-solvent shows that the stabilizing influence of the changes in the solvent structure is greater on the emergent five-coordinate cation in the transition state than that on the complex ion in the initial state, with the difference becoming greater as the mole fraction of the co-solvent increases.

Kinetic and Thermodynamic Studies of Uncatalysed and Hg(II)-Catalysed Solvolysis of a Chlorocobalt(III) Complex in Binary Aqueous Media

The kinetics of solvolysis of trans — [Co(py)4Cl2]ClO4 have been investigated in aquo-organic solvent media such as water + ethanol (0-60% v/v) and water -+ urea (0-40% w/w) in the temperature range 40-60°C. The first-order rate constant varies nonlinearly with the reciprocal of the relative permittivity ∊r. The solvolysis of these types of complex involves a rate-determining dissociative step corresponding closely to 100% separation of Co3+ … Cl in the transition state. The Gibbs energy cycle relating the Gibbs energy of activation in water and in the mixtures to the Gibbs energies of transfer of individual ionic species between water and the mixtures ΔG°t(Cl) can be applied. Moreover, more than one extrema in the enthalpies and entropies of activation are found with both binary aqueous solvent mixtures. The solvolysis of trans — [Co(py)4Cl2]ClO4 has also been studied in the presence of Hg(II) and the ionic strength dependence of the reaction rate has been analysed.

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

SummaryThe 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 (Dinfssup−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 x2 ca. 0.1 and a minimum of x2 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.

Kinetic studies of solute – solvent interactions in the solvolysis of trans -[Co(4- tert -butylpyridine) 4 Cl 2 )ClO 4 in organic-aqueous mixtures

The kinetics of solvolysis of trans-[Co(L)4 C12] C1O4 where (L-4-tert-butylpyr-idine) were followed spectrophotometrically in water-dioxane and water-dimethylsulfoxide media (0-60% v/v) over the temperature range (40-55°C). Non-linear plots were found for the logarithm of the rate constant of the first-order reaction versus the reciprocal of the relative permittivity, 3**r, of the mixed solvent. This behaviour can be attributed to the differential solvation of the initial and transition states of the complex. The enthalpies and entropies of activation showed compensating extrema with the mole fraction of solvent, leading to a small variation in the free energy of activation. By applying a free-energy cycle, it was found that the difference between the values of the free energy of transfer of the cations in the transition and initial states were negative, indicating that the cation in the transition state is more solvated than that in the initial state.