Carbonyl halides of the Group VIII transition metals. IV. Halocarbonyls and halocarbonyl anions of rhodium(III) and rhodium(I)

Both rhodium trichloride and tribromide are easily carbonylated and subsequently reduced by refluxing formic acid-hydrohalic acid mixture to give almost quantitative yields of the rhodium(1) halocarbonyl anions [Rh(CO)2X2]- (X = Cl,Br). It has been shown that the reaction proceeds by the intermediate formation of the rhodium(111) halocarbonyl anions [Rh(CO)X5]2- followed by slow reduction to the rhodium(1) complexes. Evaporation of the respective solutions leads to almost quantitative recovery of the new rhodium(111) halocarbonyls Rh(CO)X3 and the well known rhodium(1) compounds [Rh(C0)2X]2. Iododicarbonylrhodium(1) could not be isolated by this method and in fact the only products which could be isolated were triiodocarbonylrhodium(111), Rh(CO)I3, and its corresponding anion; this shows that although the carbonylation reaction had occurred the subsequent reduction did not proceed. In solution the rhodium(1) complexes [Rh(C0)2X2]- oxidize to give [Rh(CO)X5]2-. Caesium salts of these rhodium(111) anions are readily isolated, but addition of caesium salts to the rhodium(1) solutions did not give the expected Cs[Rh(CO)2X2] but instead the octahedral complexes Cs2[Rh(C0)2(H2O)X3]. All of these changes in composition and oxidation state have been followed in solution, as well as in the isolated solid products, by infrared spectroscopy.