Effect of solvent on the reactions of coordination complexes. Part 2. Kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) in methanol–water and ethylene glycol–water media

The kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) have been investigated in aqueous methanol (MeOH) and aqueous ethylene glycol (EG) media (0-80% by weight of MeOH or EG) at 45-64.7 °C. The logarithm of the pseudo-first-order rate constants for MeOH-water media exhibits linear dependence with the reciprocal of the bulk dielectric constant (D-1s), the mole fraction of MeOH (XMeOH) and the solvent ionizing power Y(Y1-AdCl) as determined by the solvolysis rates of 1-adamantyl chloride. Similar plots (log ksobsvs.xEG or D-1s) for EG-water media are non-linear. It is evident that the solvation phenomenon plays dominant role and the rate of solvolysis is mediated by the dual solvent vectors, the overall acidity and basicity of the solvent mixtures. The relative transfer free-energy calculations indicate that the mixed solvent media exert more destabilizing effect on the transition state as compared to the initial state. The activation enthalpy and entropy vs.Xorg(where Xorg is the mole fraction of the organic solvent component) plots display maxima and minima indicating that the solvent structural changes play significant role in the activation process. The activation free energy at a given temperature, however, increases only marginally and linearly with increasing Xorg. The mutual compensatory effect of activation enthalpy and entropy on the activation free energy is in keeping with the fact that the perturbations of the reaction zone and the solvent network remain approximately proportional to each other with increasing Xorg so that the isodelphic and the lyodelphic components of ΔH± and ΔS± correlate well with each other

Base-catalysed hydrolysis of cis-(imidazolato)-, cis-(benzimidazolato)- and cis-(N-methylimidazole)-bis-(ethylenediamine)halogenocobalt(III) cations. A comparison of the reactivities of the deprotonated imidazole and benzimidazole complexes

The base hydrolysis of the complexes cis-[Co(en)2B(X)]n+ [en = ethylenediamine; B = imidazole (Him), N-methylimidazole (mim), or benzimidazole (Hbzim); X = Cl or Br] has been investigated at 20-35°C and [OH-]τ=(4.7-99.7)×10-3 mol dm-3(I= 0.10 mol dm-3) under which conditions the co-ordinated imidazole and benzimidazole undergo complete NH deprotonation. The activation parameters (85≤ΔH‡/kJ mol-1 ≤ 97, +84 ≤ ΔS‡/J K-1 mol-1 ≤ +97), and the rate dependence on the leaving groups and the non-labile amine ligands are consistent with a SN1 CB mechanism. The electron-displacement properties of the N-co-ordinated imidazolate (im) and benzimidazolate (bzim) ions appear to enhance the pKNH of the co-ordinated ethylenediamine, the effect being relatively more significant for the former anion. Analysis of the activation entropy data in terms of the dissociative activation model for the conjugate bases, cis-[Co(en)(en - H)B(X)]n+[B = im or bzim (n= 0); mim (n= 1)] indicates that the configurational rearrangement at the cobalt(III) centre most likely occurs in the transition state of the actual acta of the substitution for the less reactive chloro complexes. The presumed trigonal-bipyramidal intermediate is efficiently scavenged by azide

Decarboxylation of Hydrogencarbonatopentamminecobalt(III) in Aquo-organic solvent media

The decarboxylation of hydrogencarbonatopentaamminecobalt(III) has been investigated in aqueous, 99% D2O and aquo-organic solvent media (0-70 wt.% of cosolvent) at 15 ≤ t/°C ≤ 40 (I = 0.02 mol dm-3), using methanol, propan-2-01, tert-butyl alcohol, ethylene glycol, acetone, acetonitrile, DMSO and ethylene carbonate as cosolvents. The solvent isotope effects on rate (kH2O/kD2O=O 1.0 at 1535°C) and activation parameters (ΔH≠ = 77.7 ± 1.0, 77.8 ± 0.9 kJ mol-1 and ΔS≠ = 16 ± 3, 16 ± 3 J K-1 mol-1 for aqueous and 99% D,O media, respectively) were negligible. The decarboxylation rate constant increased with increasing mole fraction (Xorg) of the cosolvent and the effect was pronounced at relatively high values of Xorg for the dipolar aprotic cosolvents. This was attributed to a greater degree of destabilisation of the initial state as compared to the transition state with increasing mole fraction of the cosolvent. The In ks vs. 1/εs plots (ks is the rate constant and εs, the bulk relative permittivity) showed marked dependence on the nature of the cosolvents; the gradients of such plots generally increased with increasing dipole moment of the cosolvent molecules, indicating thereby that the solvation of the initial state and the transition state of the substrate is governed by the ion-dipole interactions between the water and cosolvent molecules. The relative transfer free energy of activation, [ΔΔGt≠](s←w), decreased linearly with Xorg for all mixed-solvent media, indicating that the preferential solvation effect is not significant. The activation enthalpy and entropy vs. Xorg plots displayed extrema suggesting that these thermodynamic parameters are sensitive to the structural changes in the bulk solvent phase. The solvent effects on ΔH≠ and ΔS≠ are mutually compensatory

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 ksaq vs. Ds-1 (where Ds is the bulk-dielectric constant of the solvent mixture) and log ksaq 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 ksaq vs. molfraction of methanol (XMeOH) 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 XMeOH exhibit maxima at XMeOH = 0.06, 0.27, and 0.12, 0.36 and minima at XMeOH = 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 XMeOH 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

Medium effects on the electron-transfer processes: Reduction of aquapentaamminecobalt(III) by hexacyanoferrate(II) in ethyleneglycol-water, ethylene carbonate-water and urea-water media

824-831The outer-sphere electron transfer between Co(NH3)5OH and Fe(CN) - which proceeds via precursor ion-pair formation has been investigated in ethylene glycol-water (0-47.6 wt% of EG), ethylene carbonate-water (0-40 wt% of EC) at 20-35ᵒC and in urea-water (0-19.4 wt% of EG) at 25ᵒC (I = 0.50 mol dm-3 NaNO3)· The electron transfer rate constant for the ion pair (k, s-1) follows the order H2O ~ U-H2O 2O 2O. The specific solvent effect on the rate constant for EGH2O and EC-H2O media can be account d for in the frame work of the Marcus theory by considering the ion solvent dipole interaction in the solvent cospheres of the precursor and the successor ionpairs. Both H≠ and S≠ decrease linearly with increase in mole fraction of EC(XEC) while for the EG-H2O media a broad maximum s discernible in the plot of H≠ (or S≠)­ versus XEG indicating thereby that the solvent structural effects might mediate the activation process

Solvent Effects on Reactions of Coordination Complexes: Part IV – Acid Catalysed Aquation of Oxygen Bonded (ɑβS)-(Sulphito)(tetraethylenepent-amine)cobalt(III) in Aqeous Binary Mixtures of Protic & Dipolar Aprotic Consolvents-A Flash Photolysis Study on Sulphur Bonded (ɑβS)(Sulphito)-(tetraethylenepentamine)cobalt(III) Ion

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