The partial molar volumes of aqueous metal cations: their prediction and relation to volumes of activation for water exchange

Abstract

Partial molar volumes [Formula: see text] of the Fe2+, Ni2+, Fe3+, Cr3+, Rh3+, and M(NH3)5OH3+ (M = Cr, Co, Rh, Ir) aqueous ions have been obtained by extrapolation from density measurements at 298.15 K on the perchlorate salt solutions containing added HClO4 where necessary to suppress hydrolysis or oxidation. The values for these high-spin aqua-ions and others in the literature can be represented satisfactorily for ionic charges z = 1 through 4 by[Formula: see text]where [Formula: see text] is the absolute partial molar volume in cm3 mol−1 of Mz+(aq) relative to [Formula: see text] for H+(aq), and r is the Shannon radius in pm of Mz+ with coordination number nc. Volumes of activation ΔVex* for water exchange correlate inversely with [Formula: see text] within the series M2+(aq) and M(NH3)5OH23+(aq) at least, allowing predictions of ΔVex* to be made from [Formula: see text] or the above equation. The physical origin of these correlations is considered.