Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, where n varies from 0 to 3. These are diamagnetic red-brown solids. The soluble trihydrated (n = 3) salt is the usual compound of commerce. It is widely used to prepare compounds used in homogeneous catalysis.[7]
Structures
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, it 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.[8] The anhydrous material is insoluble in common solvents and, for that reason, of little value in the laboratory.
Hydrates and aqueous solutions
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].[9] These species have been separated by ion exchange chromatography and individually characterized by UV-vis spectroscopy.[10]
Preparation
RhCl3(H2O)3 is produced from salts such as Na3RhCl6, the latter being obtained in the purification of rhodium from the other platinum group metals such as platinum and iridium. The trisodium salt is converted to H3RhCl6 by ion exchange chromatography. Recrystallization of this acidic salt from water affords the hydrated trichloride, sometimes called "soluble rhodium trichloride."[11] Anhydrous RhCl3 is prepared by reaction of chlorine with rhodium sponge metal at 200–300 °C.[12] Above 800 °C, the anhydrous chloride reverts to Rh metal and chlorine.[11]
Coordination complexes
Despite the complexity of its solutions, hydrated rhodium trichloride is the precursor to a wide variety of complexes prepared in high yields.These complexes generally arise by substitution reactions, whereby of water and chloride are replaced by more basic ligands as described in the sections below. These reactions are facilitated by the fact that hydrated rhodium trichloride is soluble in a range of polar organic solvents.
Oxygen and nitrogen-based ligands
Evidence for the affinity of rhodium chlorides for oxygen-based ligands is provided by the chloro-aquo complexes discussed above. Rhodium trichloride reacts with acetylacetone to give rhodium acetylacetonate.
Aqueous solutions of rhodium trichloride react with ammonia to give the salt pentamminerhodium chloride, [RhCl(NH3)5]Cl2. As for other metal-ammine complexes, the term "ammine" refers to ammonia bound to a metal ion as a ligand. Zinc reduction of this cation followed by the addition of sulfate gives the colourless hydride complex [HRh(NH3)5]SO4.[13] Some rhodium ammine chlorides are used in the purification of rhodium from its ores.[14]
Upon boiling in a mixture of ethanol and pyridine (py), hydrated rhodium trichloride converts to trans-[RhCl2(py)4)]Cl. In the absence of a reductant, the reaction affords fac-[RhCl3(py)3], analogous to the thioether derivatives.[8] Oxidation of aqueous ethanolic solution of pyridine and RhCl3(H2O)3 by air affords a blue paramagnetic oxygen-bridged compound, [Cl(py)4Rh-O2-Rh(py)4Cl]5+.[15]
Thioethers and tertiary phosphines
Rhodium(III) also forms a range of complexes with soft Lewis bases, such as thioethers, phosphines, and arsines. Such ligands form Rh(III) complexes, but unlike the "hard" N- and O-based ligands, these complexes often can be reduced to Rh(I) derivatives. The reactions are facilitated by the solubility of rhodium trichloride in alcohols, which also dissolve the organic ligands. Thus, ethanolic solutions of hydrated rhodium trichloride react with diethyl sulfide:[16]