Synthesis and characterization of rhodium(III)–chromium(III) heterotrinuclear aqua ions
Abstract
Two new heterotrinuclear aqua ions, [Rh2Cr(µ-OH)4(H2O)9]5+ and [RhCr2(µ-OH)4(H2O)9]5+, have been prepared by treating chromium(III) dimer with Rh3+ and Cr3+ with rhodium(III) dimer in alkaline solution and isolated as pure solutions by ion-exchange chromatography. Formation of the heterometallic aqua ions has been confirmed through various solution characterization studies and analyses of the ‘active’ hydroxides derived from them. These heterotrinuclear aqua ions are proposed to be generated through nucleophilic attack by OH groups attached to the rhodium(III) aqua ion, formed by deprotonation of terminal aqua ligands, at the more labile chromium(III) centres. Acid cleavage of [RhCr2(µ-OH)4(H2O)9]5+ gave the parent mononuclear aqua ions Rh3+ and Cr3+ in a 1∶2 ratio by a process which is first order in both [aqua ion] and [H+]. The acid dependence of the reaction rate reflects the existence of acid-independent [kuc = 2.4 × 109 s–1, ΔH‡ = 132(3) kJ mol–1; ΔS ‡ = +32(7) J K–1 mol–1] and -dependent [kc = 2.8 × 109 dm3 mol–1 s–1, ΔH ‡ = 150(1) kJ mol–1; ΔS ‡ = +93(4) J K–1 mol–1] cleavage pathways. The activation parameters for the acid-independent process indicate an Id mechanism which becomes more associative on protonation of the aqua ion.