EFFECT OF SOLVENT ON THE REACTIONS OF COORDINATION-COMPLEXES - PART 23 - KINETICS OF BASE HYDROLYSIS OF SOME ONOCARBOXYLATO)(TETRAETHYLENEPENTAMINE)COBALT(III) COMPLEXES IN DIMETHYL-SULFOXIDE PLUS WATER MEDIA

The base hydrolysis of [(tetren)CoO2CR](2+) {tetren = tetraethylenepen tamine; R = -CH2NH2, -CH2CH2NH2, -(2-)C5H4N (alpha beta S isomer), -CH (CH3)N-2 (alpha beta R and alpha beta S isomers)} has been investigate d in DMSO + water (0 <X-DMSO < 0.274, X-DMSO = mole fraction of DMSO) at 20.0 < t/degrees C < 40.0. The destabilization of OH- in mixed solv ent media alone does not explain the observed rate acceleration. The s olvent composition dependence, log(k)OH(OS) = log(k)OH(OW) + Sigma a(i )X(DMSO)(i)(i=1,2,k(OH)(o) denote k(OH) at I = 0 in mixed solvent(s) a nd water (w) indicates specific solute solvent interactions explicable in terms of solvent acidity (alpha(s)), basicity (beta(s)) and polari zability (pi(s)). The values of the relative transfer free energy dat a [Delta(t)G(t.s.)- Delta(t)G(o)(i.s)](s<--w))(25 degrees C) (=Y), whe re t.s. and i.s. denote transition state and initial state of the subs trates, are positive for all substrates at all compositions indicating greater destabilising effect of the mixed solvent on the trnsition st ate than on the initial state. The Y values have a negative correlatio n with G(E) (Y - a X-DMSO + cG(E), both c and G(E) negative) for all a ps isomers, which presumably correspond to a positive destabilising ef fect of the solvent structure on both t.s. and i.s., the effect being relatively, larger on the former. The variation of the relative transf er enthalpy and entropy values (Delta Delta X(t.s) - (i.s)](s<--w)) (X =H, S) with X-DMSO further supports that solvent structural effects me diated the rates and energetics of the reaction. However, the solvent effects on the solvation components of Delta H-# and Delta S-# are mut ually compensatory (Delta H-#/kJ mol(-1) = (57.4+/-3.1) + 0.333+/-0.02 6) Delta S-#/JK(-1) mol(-1)) this indicating that there is no change i n the mechanism.