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

Abstract

For the replacement of X − by H 2 O in Cr(NH 3 ) 5 X 2+ , ΔV o * (the value of the volume of activation ΔV* at zero pressure) at 25.0° in 0.1 m NH 4 ClO 4 is −10.8, −10.2, and − 9.4 cm 3 mol −1 respectively for X − = Cl − , Br − , and I − , while ΔV, the molar volume of reaction, is −8.4, −7.2 and −6.0 cm 3 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 cm 3 mol −1 for the aquo-exchange of Cr(NH 3 ) 5 OH 2 3+ , indicate an associative mechanism for these reactions, as does the contrast with ΔV 0 * data for the formally analogous, but dissociatively-activated, reactions of Co(NH 3 ) 5 X (3−n)+ .