Pressure Effects upon the Rates of Aquation of Halopentaamminechromium(III) Ions: Evidence for an Associative Mechanism

Abstract

For the replacement of X− by H2O in Cr(NH3)5X2+, ΔVo* (the value of the volume of activation ΔV* at zero pressure) at 25.0° in 0.1 m NH4ClO4 is −10.8, −10.2, and − 9.4 cm3 mol−1 respectively for X− = Cl−, Br−, and I−, while ΔV, the molar volume of reaction, is −8.4, −7.2 and −6.0 cm3 mol−1. The pressure-dependence of ΔV* can be expressed in terms of the number x of water-molecules which are electrostricted as the systems go from ground-state to transition-state; for X− = Cl−, Br−, and I−, x = 1.9, 2.0, and 1.7. These data, combined with ΔV* = −5.8 cm3 mol−1 for the aquo-exchange of Cr(NH3)5OH23+, indicate an associative mechanism for these reactions, as does the contrast with ΔV0* data for the formally analogous, but dissociatively-activated, reactions of Co(NH3)5X(3−n)+.