Part 2.-Kinetics of Solvolysis of cis-(Chloro)(imidazole)bis(ethylenediamine)cobalt (111) and cis-( C hloro)( benzimidazole) bis( e t hy1enediamine)co bal t ( 111) in Methanol-Water and Ethylene Glycol-Water Media
The kinetics of the solvolytic aquation of cis-(bromo)(benzimidazole)bis-(ethylenediamine)cobalt(m) have been investigated at 35, 40, 45, 50.2 and 54.8 "C in aqueous methanol media of methanol content 0-80 wt YO. The pseudo-first-order rate constant decreases with increasing methanol content. Plots of logk& us. 0 ; ' (where D, is the dielectric constant of the solvent mixture) and log Pw us. the Grunwald-Winstein Y parameter or revised Y,, (1-adamantyl Chloride) parameter are non-linear. The plot of log Pw us. the molefraction of methanol (XMeoH), which deviates from linearity at 35, 40 and 45 "C, is an excellent straight line (r = 0.998) at 50 and 54.8 "C over
The kinetics of the solvolytic aquation of cis-(Bromo) (imidazole) bis(ethy1enediamine) cobalt (111) and cis-(Bromo) (N-methylimidazole) bis(ethy1enediamine) cobalt(II1) have been investigated in aqueous methanol media with methanol content 0-80% by weight and a t temperatures 40-55°C. The pseudo-first order rate constant decreases with increasing methanol content. Plots of log k$ vs. D;' (where D, is the bulk-dielectric constant of the solvent mixture) and log J Z : ,~ 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,,,,) 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. Plots of activation enthalpy and entropy against molfraction of the solvent mixtures exhibit maxima and minima. This type of variations of the activation parameters, AHf and AS", 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.
The kinetics of base hydrolysis of the cis-(chloro)-(benzimidazole)bis(ethylenediamine) cobalt(II1) ion was investigated in Methanol-water and Ethyleneglycol-water media of varying compositions and at isodielectric conditions (Ds = 61 at 25°C) in isopropanol-and Acetonitrilewater media at 35°C in the pH range, 7.2-8.9. It is of interest to note that the aquation rate constant of the conjugate base, cis-[Co(en)z(bzm)Cl]+ (Bzm = N -H deprotonated benzimidazole, en = ethylenediamine) is virtually insensitive to the compositions of the mixed solvents studied, unlike the aquation rate constant of the conjugate acid form of the title complex. This is indicative of a true dissociative mode of activation (Deb) of the conjugate base.
The decarboxylation of hydrogencarbonatopentaamminecobalt(iii) has been investigated in aqueous, 99% D, O and aquo-organic solvent media (0-70 wt.% of cosolvent) at 15 d 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 a s cosolvents. The solvent isotope effects on rate ( k H z O / k D z O = 1.0 at 1535°C) and activation parameters (AH' = 77.7 & 1.0, 77.8 0.9 kJ mol-' and ASf = 16 & 3, 16 & 3 J K-' mol-' for aqueous and 99% D, O media, respectively) were negligible.T h e decarboxylation rate constant increased with increasing mole fraction (Xorg) of t h e cosolvent and t h e effect was pronounced at relatively high values of Xorg for t h e 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 k, vs. 1/~, plots (k, is t h e rate constant and E, t h e bulk relative permittivity) showed marked dependence on t h e nature of t h e cosolvents; t h e gradients of such plots generally increased with increasing dipole moment of t h e cosolvent molecules, indicating thereby that t h e solvation of t h e initial state and t h e transition state of the substrate is governed by t h e ion-dipole interactions between the water and cosolvent molecules. The relative transfer free energy of activation, [AAGc](s+w), decreased linearly with X,,, for all mixed-solvent media, indicating that t h e 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 AHf and ASf are mutually compensatory.
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