Effect of cosolvent on the volume of activation for base hydrolysis of tris(1.10-phenanthroline)- and tris (2.2 '-bipyridine)iron(II) complexes

The kinetics of the base hydrolysis of Fe(phen)(3)(2+) and Fe(bipy)(3)(2+) (phen = 1,10-phenanthroline and bipy = 2,2'-bipyridine) in some aqueous alc ohol mixtures at ambient and elevated pressures (up to 1kbar) have been mon itored spectrophotometrically at 25.0 degrees C. For a given pressure, the alcohol cosolvent increases the. rate of reaction relative to the reaction in a wholly aqueous medium. In all cases, increasing pressure causes rate r etardation and derived volumes of activation for the reactions in aqueous s olvent mixtures vary between +15 and +25 cm(3) mol(-1), indicating that sol vation changes of a different magnitude occur upon reaching the transition state from those occurring for the reactions in aqueous medium. Since the r eaction has been established earlier to be nucleophilic attack of the incom ing hydroxide ion, the volumes of activation signify marked increases in th e loss of electrostricted solvent from the vicinities of the hydroxide ion and the iron(II) complex ions. (C) 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 263-270, 2000.