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, H ‡ , and the entropy of activation, S ‡ , have been studied. Analysis of the solvent effect confirmed a common I d mechanism for the aquation of the cobalt(III) complex. C

Kinetic studies of solvent structure effects and solute-solvent interactions on the solvolysis of [Co(NH 3 ) 5 Cl] 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 ( H # and S # ) 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. C 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: [416][417][418][419][420][421][422] 2008

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The rate of aquation of bromopentaammine cobalt(III) ion in the presence of different types of dicarboxylate solutions containing tert-butanol (40% V/V) have been measured spectrophotometrically at different temperatures (30-600°C) in the light of the effects of ion-pairing on reaction rates and mechanism. The thermodynamic and extrathermodynamic parameters of activation have been calculated and discussed in terms of solvent effect on the ion-pair aquation reaction. The free energy of activation ∆Gip* is more or less linearly varied among the studied dicarboxylate ion-pairing ligands indicating the presence of compensation effect between ∆Hip* and ∆Sip*. Comparing the kip values with respect of different buffers at 40% of ter-butanol is introduced.

The experimental kinetic study of aquation for both complexes bromopentaammine cobalt(III) and chloropentaammine cobalt(III) ions in the presence of tartarate solution in mixed solvent media of water with tert-butanol (10-50%, v:v) was examined spectrophotometrically at different temperatures (30-60 °C) by comparing the special effects of the leaving group of chloro and bromo on the rate constant of aquation. Comparison of kip (rate constant of ion-pairing) for both complexes and show the non-linear plots of log (kip) ion-pair rate constants against the reciprocal of the dielectric constant D. The thermodynamic analyses of the kinetic data for both complexes have been discussed in terms of solvent effect on the ion-pair aquation reactions. The obtained isokinetic temperatures of these systems indicate the existence of compensation effect arising from solute-solvent interaction. The excessive change of ΔHip* and ΔSip* with the mole fraction of the co-solvent can be recognized to the change of the physical properties of the solvent-water mixture with the solvent structure. Undersized changes in ΔGip* with the mole fraction of the co-solvent was found, representing a compensating effects between ΔHip* and ΔSip*.