Kinetics of the solvolysis of the trans-chloroazidobis(diaminoethane)cobalt (III) ion in water + t-Butyl Alcohol Mixtures

The kinetics of the solvolysis of the ion trans-[Coen₂N₃Cl]⁺ have been investigated at several temperatures in mixtures of water with t-butyl alcohol with concentrations of the latter ranging up to 50 vol% or a mol fraction of 0.16. Values for the enthalpy and entropy of activation show sharp changes with changing solvent composition which can be correlated with extrema in the physical properties of the mixture concerned with sharp changes in solvent structure. Plots of log(rate constant) against the reciprocal of the dielectric constant and against the Grunwald-Winstein Y factor are both curved. The application of a free energy cycle shows that the effect of changes in solvent structure on the solvolysis dominates on the cobalt(III) cation in the transition state over that on the cation in the initial 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 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 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 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.     

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 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 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.     

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 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 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.     

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     

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.     

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     

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 H and entropy S of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of H versus S 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.     

The kinetics of the solvolysis of chloropentacyanocobaltate(III) ions in water containing 2-methoxyethanol or diethylene glycol: A contrast with the effect of more hydrophobic cosolvents

The kinetics of the solvolysis of [Co(CN)₅Cl]^3– have been investigated in water +2-methoxyethanol and water + diethylene glycol mixtures. Although the addition of these linear hydrophilic cosolvent molecules to water produces curvature in the variation of log(rate constant) with the reciprocal of the dielectric constant, their effect on the enthalpy and entropy of activation is minimal, unlike the effect of hydrophobic cosolvents. The application of a Gibbs energy cycle to the solvolysis in water and in the mixtures using either solvent-sorting or TATB values for the Gibbs energy of transfer of the chloride ion between water and the mixture shows that the relative stability of the emergent solvated Co(III) ion in the transition state compared to that of Co(CN)₅Cl^3– in the initial state increases with increasing content of cosolvent in the mixture. By comparing the effects of other cosolvents on the solvolysis, this differential increase in the relative stabilities of the two species increases with the degree of hydrophobicity of the cosolvent.List of Symbols v₂ partial molar volume of the cosolvent in water + cosolvent mixtures - V ₂ ^o molar volume of the pure cosolvent - H _mix ^E excess enthalpy of mixing water and cosolvent - S _mix ^E excess entropy of mixing water and cosolvent - G _t ^o (i)_n the Gibbs energy of transfer of speciesi from water into the water + cosolvent mixture excluding electrostatic contributions - k _s first order rate constant for the solvolysis in water + cosolvent mixtures - D _s dielectric constant of the water + cosolvent mixture - H ^* the enthalpy of activation for the solvolysis - S ^* the entropy of activation for the solvolysis - G ^* the Gibbs energy of activation for the solvolysis - V ^* the volume of activation for the solvolysis - i ^* speciesi in the transition state for the solvolysis - H _o Hammett Acidity Function - TATB method for estimating the Gibbs energy of transfer for single ions assuming those for Ph₄As⁺ and BPh ₄ ^– are equal     

Effect of solvent and structure on the kinetics and mechanism of the aquation of bromopentaamine cobalt(III) complex in binary aqueous mixtures

The kinetics of aquation of bromopentaamine cobalt(III) complex have been investigated spectrophotometrically in aqueous‐organic solvent media using acetonitrile, urea, and dimethyl sulfoxide as co‐solvents at 45 ≤ T (°C) ≤ 65. The logarithms of rate constant of the aquation reaction vary nonlinearly with the reciprocal of the dielectric constant for all cosolvent mixtures, indicating a specific solute–solvent interaction. Also, the rate constants are correlated with the total number of moles of water and the organic solvents. However, the solvent effects on the solvation components of the enthalpy of activation, ΔH^?, and the entropy of activation, ΔS^?, have been studied. Analysis of the solvent effect confirmed a common I_d mechanism for the aquation of the cobalt(III) complex. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36:494–499, 2004     

Effect of solvent on the reactions of coordination complexes part VII: Kinetics of solvolysis of cis-(bromo) (imidazole) bis-(ethylenediamine)cobalt (III) and cis-(bromo) (N-methyl imidazole)bis-(ethylenediamine) cobalt(III) in methanol-water media

The kinetics of the solvolytic aquation of cis-(Bromo) (imidazole) bis(ethylenediamine) cobalt (III) and cis-(Bromo) (N-methylimidazole) bis(ethylenediamine) cobalt(III) have been investigated in aqueous methanol media with methanol content 0–80% by weight and at temperatures 40–55°C. The pseudo-first order rate constant decreases with increasing methanol content. Plots of log k vs. D (where D_s is the bulk-dielectric constant of the solvent mixture) and log k vs. the Grunwald-Winstein Y-solvent parameter are nonlinear, the curvature of the plots is relatively more significant for the imidazole complex. The plots of log k vs. molfraction of methanol (X_MeOH) for both the substrates also deviate from linearity, the deviation being less and less marked, particularly for the N-methyl imidazole complex, as the temperature is increased. Hence preferential solvation phenomenon appears to be less significant when the N-H proton of imidazole is replaced by -CH₃ group. The plots of calculated values of the transfer free energy of the dissociative transition state, cis-{[(en)₂Co(B)]³⁺}* (B = imidazole, N-methylimidazole), relative to that of the initial state, cis-[Co(en)₂(B)Br]²⁺, for the transfer of the ions from water to the mixed solvent, against X_MeOH exhibit maxima at X_MeOH = 0.06, 0.27, and 0.12, 0.36 and minima at X_MeOH = 0.12 and 0.19 for cis-[(en)₂Co(imH)Br]²⁺ and its N-methylimidazole analogue respectively which are in keeping with the solvent structural changes around the initial state and transition state of these substrates as the solvent composition is varied. Plots of activation enthalpy and entropy against molfraction of the solvent mixtures exhibit maxima and minima. This type of variations of the activation parameters, ΔH^≠ and ΔS^≠, with X_MeOH speaks of the enthalpy and entropy changes associated with the solvent-shell reorganization of the complex ions both in the initial and in the transition states which contribute appreciably to the overall activation enthalpy and entropy of the aquation reaction.     

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.