Rhodium(III) iodide is an inorganic compound with the formula RhI3. It is a black solid. Some rhodium(III) iodides are used as commercial catalysts.
Structure RhI3 adopts same crystal structure motif as AlCl3 and YCl3. The structure consists of cubic close-packed iodide ions and rhodium ions filling a third of the octahedral interstices, forming a layers.
Preparation Rhodium(III) iodide can be synthesised by the reaction of aqueous potassium iodide with rhodium(III) bromide.
RhBr3 + 3KI → RhI3 + 3KBr
Reactivity Rhodium(III) iodide is only known in the anhydrous form. Unlike the other rhodium(III) halides, it does not form hydrates. The related anion [RhI6]3− was previously thought not to form but has since been prepared by diffusion of RhCl3·3H2O through a layer of hydroiodic acid into piperazine.
Although RhI3 and [RhI6]3− remain laboratory curiosities, other rhodium(III) iodides have had a major impact technologically. They are catalysts used in the Monsanto process, which at one time was the dominant method for producing acetic acid, a commodity chemical. One intermediate in that cycle is [(CH3)Rh(CO)2I3]−.Hosea Cheung, Robin S. Tanke, G. Paul Torrence "Acetic Acid" in Ullmann's Encyclopedia of Industrial Chemistry, 2002, Wiley-VCH, Weinheim. .
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