Medium effect on the reactions of coordination complexes: Base hydrolysis of (#alpha##beta#S)(p-hydroxybenzoato)(tetraethylenepentamine)-cobalt(III) in methanol-water, acetone-water and acetonitrile-water media

摘要

The kinetics of base hydrolysis of (#alpha##beta#S)(p-hydroxybenzoato)(tetraethylenepentaamine)cobalt(III) has been studied in aqua-organic solvent media using MeOH, Me_2CO and MeCN as cosolvents at 20.0 <= t~0 C<= 40.0 (I= 0.02 mol dm~(-3)) with 80% (v/v) of cosolvents. Only the base catalyzed path (k_obs = k_OH [OH~-]) has been observed.The relative second order rate constant, k_OH/k_OH~0w at I=0, increases nonlinearly with increase in mole fraction (x_o.s.) of the cosolvents. MeCN and Me_2CO exert much stronger rate acceleration than MeOH. The relative transfer free energy [#DELGA#_tG(t.s.)-#DELTA#G(rt.s.)]_(s<-w)~298K is positive for MeCN and negative for both Me_2CO and MeCN indicating greater destabilising effect of the solven ton the transition state (t.s.) relative to the initial state(i.s.). for MeCH-H_2O while the effect is reversed for MeOH-H_2O and Me_2CO-H_2O media. The variation of activation parameters (#DELTA#H and #DELTA#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.