Variable-pressure kinetic study of bromide complexation on aquarhodium(III)

A variable-pressure kinetic study of the complexation of bromide with Rh3+(aq) (I = 2.03 mol dm−3 (NaClO4−), range 0.1–206.8 MPa) as a function of [H+] (0.06–1.00 mol dm−3) at 70 °C is reported. The final product is trans-[Rh(OH2)4Br2]+ with formation of the bromopentaaqua ion rate determining. The decrease in the extent of rate saturation with [Br−] observed on lowering [H+] below 0.20 mol dm−3 is consistent with greater participation from the more labile pentaaquahydroxo ion in a parallel ion-pair interchange processes with the hexaaqua ion but with Kos (RhOH2+–Br−) < Kos (Rh3+–Br−). Complexation on the hexaaqua ion is associatively activated (ΔV‡I = −3.3 ± 1.0 cm3 mol−1) but dissociatively activated on the pentaaquahydroxo species (ΔV‡IOH = +7.7 ± 1.0 cm3 mol−1). These findings support the recent ab-initio calculations by Rotzinger et al. and variable-pressure kinetic study of the water exchange process by Merbach et al. which indicate a favourable Ia pathway for low-spin [Rh(OH2)6]3+ despite its t2g6 configuration. Earlier proposals of a D process for both ions on the basis of the observed rate saturation can now be discounted and the present findings lend further compelling support for the use of the activation volume as a powerful mechanistic probe. Reasons for the greater penetration by the entering donor within replacement reactions on 4d and 5d M3+ complexes are considered.