Kinetics of the solvolysis of thetrans-dichlorobis(1,2-diaminoethane) cobalt(III) in water + ethanonitrile mixtures

Summary The kinetics of the solvolysis of thetrans-[Coen₂Cl₂]⁺ cation (en = 1,2-diaminoethane) have been investigated over a range of temperatures in water with added ethanonitrile which has much less effect on the solvent structure than the acohols already used as co-solvents with water for this solvolysis. However, the non-linear relationship obtained for the variation of log (rate constant) with the reciprocal of the dielectric constant at constant temperature shows that the effect of changing solvent structure is still important, although, as expected from the absence of pronounced extrema in the variation with composition in water + ethanonitrile of physical properties which are influenced by such changes in solvent structure, the enthalpy and entropy of activation for the solvolysis vary smoothly with composition. 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.     

Kinetics of the solvolysis of the cis-chloroazidobis-(1,2-diaminoethane)cobalt(III) ion in water+t-butanol mixtures

Summary Rate constants for the solvolysis of the complex ioncis-[Coen₂N₃Cl]⁺ have been determined for a range of temperatures in mixtures of water witht-butanol ranging up to 50% v/v of the latter. Linear plots of log (rate constant) against the reciprocal of the absolute temperature are obtained at all concentrations oft-butanol and the variations of the enthalpy and entropy of activation with solvent composition show extrema at compositions where extrema occur in the physical properties of the mixture which are influenced by solvent structure. The importance of solvent structure is shown by the curved plot found for log (rate constant) against reciprocal of dielectric constant at constant temperature. The application of a free energy cycle to the free energies of activation in water and in the mixture and the free energies of transfer of individual ionic species between water and the mixtures shows that the effect of changes in solvent structure on the reacting cation in the transition state dominates over that in the initial state.     

Kinetics of the solvolysis of [Co(Rpy)4Cl4]+ with R = 4-t-Bu, 3-Me or 3-Et in mixtures of urea with water

Summary The kinetics of the solvolysis of complex ions trans-[Co(Rpy)₄Cl₂]⁺, with R = 4-t-Bu, 3-Me and 3-Et, have been investigated in mixtures formed by adding urea to water, 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 [Co(Rpy)₄Cl₂]⁺ 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 Co³⁺ cation in the transition state is more stable than the Co³⁺ cation in the initial state in the water + urea mixtures.     

Kinetics of the solvolysis of trans-dichlorotetra(4-t-butylpyridine)-cobalt(III) ions in mixtures of water with methanol and with ethanol

For the solvolysis of Co(4-t-Bupy)₄Cl₂^? ions in water + methanol and water + ethanol, log (rate constant) does not vary linearly with the reciprocal of the dielectric constant. The Gibbs free energy, the enthalpy, and the entropy of activation are insensitive to changes in the solvent composition in these mixtures, although a slight broad maximum in ΔH* and ΔS* probably exists at mole fractions of about 0.2 in water + ethanol. This contrasts with the extrema in ΔH* and ΔS* found with more hydrophobic alcohols used as cosolvents. However, the application of a Gibbs energy cycle to the solvolysis in water and in the mixtures shows that there is a differential effect of changes in solvent structure on the emergent solvated Co^III cation in the transition state and on Co(4-t-Bupy)₄Cl₂⁺ in the initial state. The stability of the former increases relative to that of the latter as the cosolvent content of the mixture rises. © 1995 John Wiley & Sons, Inc.     

Spectrometric study of solute–solvent interactions for the solvolysis of chloropentaamine cobalt(III) complex in binary aqueous mixtures

Kinetic studies of solvent structure effects and solute–solvent interactions on the solvolysis of [Co(NH₃)₅Cl]²⁺ complex ion have been investigated spectrophotometrically in binary aqueous mixtures. Three cosolvents were used (acetonitrile, dimethylsulfoxide, and urea) over a wide range of temperatures. Nonlinear plots were found for log(rate constant) against the reciprocal of the relative permitivity of the medium. The enthalpy and entropy of activation (ΔH^# and ΔS^#) exhibited extrema in the same composition region where the physical properties indicate sharp changes in the structure of the solvent, confirming that the solvent structure is an important factor in determining the solvolytic reactivity. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 416–422, 2008     

Kinetics of the solvolysis of the trans-dichlorobis(1,2-diaminoethane)-cobalt(III) Ion in water + urea mixtures

Following the kinetic investigation of the solvolysis of a range of cobalt(III) complexes in mixtures of water + cosolvent wherethe cosolvent enhances the solvent structure and decreases the dielectric constant, kinetic data are now reported for such a solvolysis in water + urea where urea acts as a structure breaker and enhances the dielectric constant. A plot of log (rate constant) against reciprocal of the dielectric constant shows that differential effects of changes in solvent structure occur between the initial and the transition states and, as in theinvestigations using structure-enhancing cosolvents, the principal effect of change in solvent structure on the cobalt(III) cation occurs on the penta-coordinated ion in the transition state.     

Kinetics of the solvolysis of chloropentacyanocobaltate(III) ions in water + 2-propanol mixtures

Rates of solvolysis of [Co(CN) ₅ Cl] ^3– have been determined in a range of water-rich water + 2-propanol mixtures over a range of temperatures and they show no simple correlation with dielectric constant. The variation of the enthalpy and entropy of activation with solvent composition show broad extrema and these are discussed in relation to the physical properties of the media. The application of a free energy cycle to the dissociative loss of the chloride ion in the transition state shows that the effect of changes in solvent structure as the alcohol content increases is to stabilize the emergent pentacyanocobaltate(III) ion Co(CN) ₅ ^2– relative to the chloropentacyanocobaltate(III) anion in the initial state.     

Kinetics of the solvolysis of chloropentacyanocobaltate (III) ions in mixtures of ethane-1,2-diol or ethanonitrile with water

The kinetics of the solvolysis of [Co(CN)₅Cl]^3? ions have been investigated in mixtures formed by the addition of ethane-1,2-diol or ethanonitrile to water where the physical properties indicate little enhancement of structure in water-rich conditions. The effect on the kinetics of this solvolysis of this lack of structural change in the solvent shows in the linearity of the variation of log (rate constant) with the reciprocal of the dielectric constant for the addition of ethane-1,2-diol to water and the absence of any prominent extrema in the enthalpy or entropy of activation using either co-solvent. However, the effect of changes in solvation on the solvolysis with these two co-solvents appears to operate in a similar manner to the effects found when the hydrophobic co-solvent propan-2-ol is added to water. The application of a free energy cycle to the process of the initial state going to the transition state for this dissociative process suggests that, with both co-solvents, [Co(CN)₅]^2? in the transition state is more stable than [Co(CN)₅Cl]^3? in the initial state. © John Wiley & Sons, Inc.     

Kinetics of the solvolysis of a cationic and of an anionic chloro-cobalt(III) complex in water-propan-1-ol mixtures

Summary The kinetics of the solvolysis of the ions [Co(NH₃)₅Cl]²⁺ and [Co(CN)₅Cl]^3– have been investigated in a range of water-rich mixtures of water with n-PrOH at various temperatures. The variation of the 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 structure, 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 solvolyses of both complexes that the species of Co^III in the transition state is more stable in water + n-PrOH mixtures than the ionic complexes of Co^III in the initial state. The results are compared with those for the solvolysis of these and related complexes in mixtures of water with other alcohols.     

Kinetics of the solvolysis of chloropenta-amminecobalt(III) ions in mixtures of water with t-butyl alcohol or with 2-methoxyethanol

Summary As the transition state for the solvolysis of [Co(NH₃)₅Cl]²⁺ ions is known to have Cl^- ions in a situation closely similar to that in the bulk solvent, the kinetics of this solvolysis have been investigated for comparison in H₂O with added cosolvents of low and high hydrophobicities. A linear variation of log(rate constant) with the reciprocal of the dielectric constant is found with the former, but not with the latter cosolvent. Maxima in the enthalpies and entropies of activation found using the more hydrophobic cosolvent appear at solvent compositions where extrema occur in the physical properties influenced by structural changes in the solvent. The application of a free energy cycle to the solvolysis in H₂O and in the mixtures shows that the emergent solvated cobalt(III) ion in the transition state is more stabilised in the latter than [Co(NH₃)₅Cl]²⁺ with both cosolvents. The application of such a cycle to cases where the initial state is destabilised in the mixture is discussed.     

Medium effects on the initial and the transition states upon the solvolysis of trans‐dichlorobis(N‐methyl‐ethylenediamine)cobalt(III) complex in water‐mixed solvents

The kinetics of solvolysis of trans‐dichlorobis(N‐methylethylenediamine)cobalt(III) complex have been investigated in aqua‐organic solvent media (0–60% (v/v) cosolvent) at 25 ≤ t°C ≤ 60, using n‐propanol and tert‐butyl alcohol as cosolvents. The first‐order rate constant increased nonlinearly with the reciprocal of the dielectric constant D_s^?1, and x_org, reflecting the individuality of the cosolvents and thereby suggesting that the relative stabilities of the transition state and initial state were governed by the preferential solvation effect. The thermodynamic parameters (ΔH^≠ and ΔS^≠) were sensitive to the structural changes in the bulk solvent phase. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 495–499, 2002     

Solvent effects on the initial and transition states upon solvolysis oftrans-dichlorotetra(4-ethylpyridine) cobalt(III) perchlorate in water +t-butanol mixtures

Summary The solvolysis of the title complex was investigated in water and in water +t-butanol mixtures in the 40 to 55°C range. The activation thermodynamic parameters were calculated and the extrema observed for H and S were compared with data obtained from the measured physical properties of the same solvent mixtures. The ethyl substituent in the pyridine rings manifests its effect on the activation entropy values and on the relatively low activation energies compared with the respective values obtained previously from the solvolysis oftrans-[Co(4-MePy)₄Cl₂]⁺ in the same media. The free energies of transfer of the cation were calculated in the ground and transition states. In the ground state, the ethyl and methyl substituents in the pyridine rings provide the complex cation with nearly equal stabilities. However, the effect of solvent on the cation stability is more pronounced in the transition state for the ethyl substituent.     

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 the solvolysis oftrans-dichlorotetra-(4-t-butylpyridine)-cobalt(III) ions in water +t-butyl alcohol mixtures

Rates of solvolysis of the complex cation [Co(4tBupy)₄Cl₂]⁺ have been determined in mixtures of water with the hydrophobic solvent, t-butyl alcohol. The solvent composition at which the extremum is found in the variation of the enthalpy H^* and the entropy S^* of activation correlates well with the extremum in the variation of the relative partial molar volume of t-butyl alcohol in the mixture and the straight line found for the variation of H^* with S^* is coincident with the same plot for water + 2-propanol mixtures. A free energy cycle is applied to the process initial state (C ⁿ⁺) going to the transition state [M⁽ⁿ⁺¹⁾⁺...Cl^–] in water and in the mixture using free energies of transfer of the individual ionic species, G _t ^o (i), from water into the mixture. Values for G _t ^o (i) are derived from the solvent sorting method and from the TATB/TPTB method: using data from either method, changes in solvent structure on going from water into the mixture are found to stabilize the cation in the transition state, M⁽ⁿ⁺¹⁾⁺, more than in the initial state, C ⁿ⁺. This is compared with the application of the free energy cycle to the solvolysis of complexes [Co(Rpy)₄Cl₂]⁺ and [Coen₂LCl]⁺ in mixtures of water with methanol, 2-propanol or t-butyl alcohol: the above conclusion regarding the relative stabilization of the cations holds for all these complexes in their solvolyses in water+alcohol mixtures using values of G _t ^o (Cl^–) from either source.     

Kinetics of solvolysis of trans-dichlorotetra (t-butylpyridine)-cobalt(III) ions in mixtures of water with ethane-1,2-diol and with 2-methoxyethanol. A contrast with the effect of hydrophobic co-solvents

The kinetics of the solvolysis of the hydrophobic ion, Co(4-t-Bupy)₄Cl₂⁺, have been followed in mixtures of water with co-solvents having a hydrophilic tendency, ethane-1,2-diol and 2-methoxyethanol. The variation of In(rate constant) with the reciprocal of the dielectric constant is nonlinear for both co-solvents. The enthalpy and the entropy of activation are rather insensitive to changes in the solvent composition in both mixtures, but low maxima may exist at mol fractions of co-solvent Ca. 0.10–0.20. The application of a Gibbs energy cycle to the process of the initial state going to the transition state suggest that, in water-rich conditions, the increase in the stability of the emergent solvated Co^III ion in the transition state relative to the increase in the stability of Co(4-t-Bupy)₄Cl₂⁺ in the initial state as the co-solvent content rises is greater for 2-methoxyethanol than for ethane-1,2-diol. © 1995 John Wiley & Sons, Inc.     

Solvent effects on the initial and transition states upon solvolysis oftrans-dichlorotetra(4-ethylpyridine)cobalt(III) perchlorate in water+methanol mixtures

Summary The solvolysis of trans-[Co(4-Etpy)₄Cl₂]⁺ (1) (Etpy=ethylpyridine) was investigated in water and in a wide range of water+methanol mixtures. The effect of temperature on the solvolysis was considered and the thermodynamic parameters of activation were calculated. The free energies of transfer of (1) in the initial state were calculated from the solubility of the complex [Co(4-Etpy)₄Cl₂]₂[ReCl₆]. The free energy of transfer of the cation (1) in the transition state was calculated using a free energy cycle. The effect of the solvent structure on (1) is dominant in the transition state, where –G _t ^o (1) transition >–G _t ^o (1) initial. Comparison of the present thermodynamic data with those obtained previously fortrans-[Co(4-Mepy)₄Cl₂]⁺, (2), andtrans-[Co(py₄Cl₂]⁺, (3) in the same solvent mixture, shows that the stability increases in the order: (1), (2)<(3) at low mole fraction of the co-solvent, contrary to expectation, while at high mole fraction the order of increasing stability is: (1)<(3)<(2). The results were interpreted on non-electrostatic grounds.     

Kinetics of the solvolysis of thetrans-dichlorobis (Diaminoethane)-cobalt(III) ion in aqueous ethylene carbonate and aqueous propylene carbonate

The kinetics of the solvolysis of trans-[Coen₂Cl₂]⁺ have been followed in mixtures of water with either ethylene carbonate or propylene carbonate over a range of temperatures. Both the enthalpy and entropy of activation for the first order loss of a chloride ion to give [Coen₂Cl]²⁺ in water + ethylene carbonate show a maximum at low mole fractions of ethylene carbonate. As similar extrema in H and S for the same process for this complex and others in water +2-propanol and in water + t-butanol correlate well with extrema in the physical properties of the mixtures which are influenced by changes in solvent structure, it is suggested that these new extrema can be attributed to solvent structure effects. The application of a free energy cycle to the loss of the chloride ion in water and in the mixtures suggests that, although changes in solvent structure influence the cation in the transition state more than the cation in the initial state in water + ethylene carbonate, in water + propylene carbonate the influence of changes in solvent structure approximately balances. This is compared with the application of the free energy cycle to the same process in mixtures of water with a range of cosolvents using kinetic data available in the literature.     

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 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.     

Kinetics of the solvolysis of [Co(3Rpy)4Cl2]+ ions (R = Me or Et) in water and in water + methanol mixtures

Rates of solvolysis of ions [Co(3Rpy)₄Cl₂]⁺ with R = Me and Et have been measured over a range of temperatures for a series of water-rich water + methanol mixtures to investigate the effect of changes in solvent structure on the solvolysis of complexes presenting a largely hydrophobic surface to the solvent. The variation of the enthalpies and entropies of activation with solvent composition has been determined. A free energy cycle relating the free energy of activation in water to that in water + methanol is applied using free energies of transfer of individual ionic species from water into water + methanol. Data for the free energy of transfer of chloride ions ΔG(Cl^?) from both the spectrophotometric solvent sorting method and the TATB method for separating ΔG(salt) into ΔG(i) for individual ions are used: irrespective of the source of ΔG(Cl^?), in general, ?ΔG(Co(Rpy)₄Cl²⁺) > ?ΔG(Co(Rpy)₄Cl₂⁺), where Rpy = py, 4Mepy, 4Etpy, 3Etpy, and 3Mepy, showing that changes in solvent structure in water-rich water + methanol mixtures generally stabilize the cation in the transition state more than the cation in the initial state for this type of complex ion. A similar result is found when the free energy cycle is applied to the solvolysis of the dichloro (2,2′,2″-triaminotriethylamine)cobalt(III) ion. The introduction of a Me or Et group on the pyridine ring in [Co(Rpy)₄Cl₂]⁺ has little influence on the difference {ΔG(Co(Rpy)₄Cl²⁺)?ΔG(Co(Rpy)₄Cl₂⁺)} in water + methanol with the mol fraction of methanol < 0.20.