Characterization of Three 'Active' Rhodium(III) Hydroxides

Abstract

Pure solutions of [ Rh (H₂O)₆]³⁺, dimer [Rh₂(μ-OH)₂(H₂O)₈]⁴⁺ and trimer [Rh₃(μ-OH)₄(H₂O)10]⁵⁺ have been converted into their respective 'active' hydroxides by dropwise addition to an imidazole solution. These 'active' hydroxides have been analysed by a variety of techniques including rhodium determination, infrared spectroscopy, thermal analysis and powder X-ray diffraction. Purity determinations using ion-exchange chromatography showed that the three hydroxides consist primarily of the neutral forms of the starting aqua ion (>96%) with small amounts of species with higher nuclearity. Rhodium analysis and thermogravimetric measurements confirmed the composition of these hydroxides to be Rh (OH)₃(H₂O)₃.H₂O, Rh₂(μ-OH)₂(OH)₄(H₂O)₄ and Rh₃(μ-OH)₄(OH)₅(H₂O)₅.5H₂O. A scheme for the thermal decomposition of each of the hydroxides has been proposed on the basis of the t.g . and d.t.a . data and the knowledge that the final product in each case is α-Rh₂O₃. Heating of the hydroxides in air resulted in oxidation of Rh^III to Rh^IV (temperature 250-300°C) forming RhO₂ which on further heating decomposed to α-Rh₂O₃ and dioxygen.