Effect of solvent on the reactions of coordination complexes: Part 23^# - Kinetics of base hydrolysis of someraminomonocarboxylato) (tetraethylenepentamine )cobalt (lII) complexes in dimethyl sulphoxide + water media

Abstract

The base hydrolysis of [(tetren)Co02CR)²⁺ {tetren = tetraethylenepentamine; R = -CH₂NH₂, -CH₂CH₂NH₂,-(2-) C₅H₄N (αβS isomer), -CH(CH₃)N₂ (αβR and abS isomers)} has been investigated in DMSO + water (0 < X_DMSO < 0.274, X_DMSO = mole fraction of DMSO) at 20.0 < t/⁰C < 40.0. The destabilization of OH in mixed solvent media alone does not explain the observed rate acceleration. The solvent composition dependence, logk_OH^OS= logk_OH^OW+ ∑ai;_DMSO(i=1,2,k_OH^o denote k_OH at I =0 in mixed solvent(s) and water (w)indicates specific solute solvent interactions explicable in terms of solvent acidity (α^s), basicity (β^s) and polarizability (π^s). The values of the relative transfer free energy data [Δ_tG(t.S.)- D_tG ^o(i.s.)]_s_!_w)(25°C) (=Y), where t.S. and i.s. denote transition state and initial state of the substrates, are positive for all substrates at all compositions indicating greater destabilising effect of the mixed solvent on the trnsition state than on the initial state. The Y values have a negative correlation with G^E (Y - a X_DMSO + cG^E, both c and G^E negative) for all abS isomers, which presumably correspond to a positive destabilising effect of the solvent structure on both t.S. and i.s., the effect being relatively larger on the former. The variation ofthe relative transfer enthalpy and entropy values (dd X(t.s) - (i.s)js •.... w) (X=H, S) with X_DMSO further supports that solvent structural effects mediated the rates and energetics of the reaction. However, the solvent effects on the solvation components of ΔH^#and ΔS^#are mutually compensatory (ΔH^# = (57.4±3.1) + 0.333±0.026) ΔS^#/JK^-1mol^-1) this indicating that there is no change in the mechanism.