Medium effect on the reactions of coordination complexes: Base hydrolysis of (αβS)(<i>p</i>-hydroxybenzoato)(tetraethylenepentamine) - cobalt(III) in methanol-water, acetone-water and acetonitrile-water media

Abstract

The kinetics of base hydrolysis of ((abS)(<em>p</em>-hydroxybenzoato)(tetraethylenepentamine)cobalt(III) has been studied in aqua-organic solvent media using MeOH, Me₂COand MeCN as cosolvents at 20.0 £ <em>t</em>^o C £ 40.0 (<em>I</em>= 0.02 mol dm^-3 ) with 80% (v/v) of cosolvents. Only the base catalyzed path (<em>k</em>_obs = <em>k</em>_OH[OH^-] )has been observed. The relative second order rate constant, <em>k</em>_OH⁰⁵ <em>I</em> <em>k</em>_OH^0w at <em>I</em> = 0, increases nonlinearly with increase in mole fraction ( <em>x</em>_o.s.) of the cosolvents. MeCN and Me₂COexert much tronger rate acceleration than MeOH. The relative transfer free energy [∆_t<em>G</em>(t.s.}- . ∆_t<em>G</em>(t.s.)](s←w)²⁹⁸ K is positive for MeCN and negative for both Me₂CO and MeCN indicating greater destabilising effect of the solvent on the transition state (t.s.) relative to the initial state(i.s.).for MeCN-H20 while the effect is reversed for MeOH-H₂O and Me₂CO-H₂O media. The variation of activation parameters (∆H^: and ∆S^:) with X_O.S.. is non-linear exhibiting extrema supporting the fact that solvent structural effects are important in controlling the energetics of the process through solvation of the initial and transition states.