SYNTHESIS AND CHARACTERIZATION OF RHODIUM(III)-CHROMIUM(III) HETEROTRINUCLEAR AQUA IONS

Two new heterotrinuclear aqua ions, [Rh2Cr(mu-OH)(4)(H2O)(9)](5+) and [RhCr2(mu-OH)(4)(H2O)(9)](5+), have been prepared by treating chromium (III) dimer with Rh3+ and Cr3+ with rhodium(III) dimer in alkaline sol ution and isolated as pure solutions by ion-exchange chromatography. F ormation of the heterometallic aqua ions has been confirmed through va rious solution characterization studies and analyses of the 'active' h ydroxides derived from them. These heterotrinuclear aqua ions are prop osed to be generated through nucleophilic attack by OH groups attached to the rhodium(III) aqua ion, formed by deprotonation of terminal aqu a ligands, at the more labile chromium(III) centres. Acid cleavage of [RhCr2(mu-OH)(4)(H2O)(9)](5+) gave the parent mononuclear aqua ions Rh 3+ 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 x 10(9) s(-1), Delta H double dagger = 132(3) kJ mol(-1); Delta S double dagger = +32(7)5 K-1 mol(-1)] and -dependent [k(c) = 2.8 x 10(9) dm(3) mol(-1) s(-1), Delt a H double dagger = 150(1) kJ mol(-1); Delta S double dagger = +93(4) J K-1 mol(-1)] cleavage pathways. The activation parameters for the ac id-independent process indicate an I, mechanism which becomes more ass ociative on protonation of the aqua ion.