A kinetic study of the aquation of fluoropentamminechromium(III) cation in aqueous solution

Abstract

The rate constants for the aquation of [Cr(NH₃)₅F]²⁺ have been measured by polarography over wide ranges of pH and temperature. In the pH range 1 to 10·5 the rate of aquation is independent of pH. The Arrhenius parameters for the aquation of [Cr(NH₃)₅F]²⁺ are compared with previously obtained data for the corresponding chloro-, bromo-, and iodo-complexes. These values are in general agreement with a bi-molecular S_N2 mechanism for aquation of the fluoro-complex and a solvent assisted S_N1 mechanism for the aquation of the other three halogeno-complexes.

The effect of added anions such as sulphate and pyrophosphate on the rate of aquation, is discussed in terms of ion-pair formation, which for the fluoro-complex produces very little increase in rate, while for the chloro-, bromo-, and iodo-complexes, appreciable catalysis is observed. This is found to be in agreement with the proposed mechanisms.