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 [k_uc = 2.4 × 10⁹ s^–1, ΔH^‡ = 132(3) kJ mol^–1; ΔS ^‡ = +32(7) J K^–1 mol^–1] and -dependent [k_c = 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 I_d mechanism which becomes more associative on protonation of the aqua ion.