Rhodium(III) chloride

Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh(III) centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt. The soluble trihydrated (n = 3) salt is widely used to prepare compounds used in homogeneous catalysis, notably for the industrial production of acetic acid and hydroformylation. Rhodium trichloride and its various hydrates can be considered the default halides of rhodium. By contrast, its lighter congener cobalt does not form a stable trichloride, mainly being available as cobalt(II) chloride. Anhydrous rhodium chloride is a dense red-brown solid. According to X-ray crystallography, tt crystallises in the motif seen also for YCl3 and AlCl3 (see image in upper right). The metal centres are octahedral, and the halides are doubly bridging. The octahedral molecular geometry adopted by RhCl3 is characteristic of most rhodium(III) complexes. The anhydrous material is insoluble in common solvents and, for that reason, of little value in the laboratory. Although hydrated rhodium trichloride is widely marketed and often used, the structure of this red solid has not been elucidated crystallographically. This reddish solid (see picture in box) is often described as RhCl3(H2O)3, but this composition has not been confirmed crystallographically. Aqueous solutions of "rhodium trichloride hydrate" have been characterized by 103Rh NMR spectroscopy. Several species are detected, the proportions of which change with time and depend on the concentration of chloride. The relative distribution of these species determines the colour of the solutions, which can range from yellow (the hexaaquo ion) to "raspberry-red". Some of these species are the aquo complexes [Rh(H2O)6]3+, [RhCl(H2O)5]2+, cis- and trans-[RhCl2(H2O)4]+, and two isomers of [RhCl3(H2O)3]. These species have been separated by ion exchange chromatography and individually characterized by UV-vis spectroscopy.