Transition-state volumes in solvent exchange. Water exchange on the aqueous aquapentaammineruthenium(III) ion

The rate of water exchange on Ru(NH₃)₅OH₂ ³⁺ in aqueous CF₃SO₃H (0.01 mol L^-1) is characterized by k^298.2 = 2.30 × 10^-4 s^-1, ΔH* = 91.5 kJ mol^-1, ΔS* = -7.7 J K^-1 mol^-1, and ΔV* = -4.0 cm³ mol^-1. The order of lability of M(NH₃)₅OH₂ ³⁺ in water exchange is (M=) Ru > Cr > Rh > Co ≫ Ir and is governed by ΔH*. For substitution reactions of metal complexes involving "hard" leaving groups, the lability sequence first < second > third transition series may be general. For a series of related aqua complexes such as M(NH₃)₅OH₂ ³⁺, variations in ΔV* for water exchange and the molar volume of the complex as M is changed are largely compensatory, so that the molar volume of the transition state varies relatively little. This reflects reduced accessibility of the complex to associative attack as the central ion becomes smaller.