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