Kinetics of Hydrolysis of cis-Difluoro-bis-(ethylenediamine)-chromium(III) Cation1

Abstract

The rates of release of fiuoride by acid and base hydrolysis of cis- STACr(en)/sub 2/F/sub 2/!/sup +/ were determined over a range of pH 1 to 8. At 25 deg the pseudo first-order aquation rate constant for loss of the first fiuoride in 0.1 f HClO/sub 4/ is 5.3 x 10/sup -6/ sec/sup -1/, much less than the corresponding rate constants of cis-STACo(en)/sub 2/F/sub 2/!/sup +/ and cis- STACr(en)/sub 2/Cl/sub 2/!/sup +/ in 0.1 f HNO/sub 3/ . The Arrhenius activation energy is 23 plus or minus 1 kcal./mole. The aquation of cis-STACr(en)/sub 2/F/ sub 2/!/sup +/ is acid catalyzed like the cobalt analog but, unlike the dichloro cobalt and chromium analogs, presumably because the difiuoro complexes aquate by a mechanism involving an intermediate reactive protonated complex. Cation- exchange and spectral evidence indicate that the aquation product is largely, if not entirely, cisSTACr(en)/sub 2/(H/sub 2/O)F!/sup 2+/. Fluoride is taken up by one or more reaction products in later stages of the hydrolysis. The aquation appears not to be accelerated by visible light. (auth)