Rhodium(III) Speciation in Concentrated Nitric Acid Solutions

Abstract

The interaction of rhodium(III) aqua ions with nitrate ions in 3–16 m nitric acid solutions has been studied by 103Rh and 15N NMR and Raman spectroscopy. The mononuclear complexes [Rh(H2O)6–n(NO3)n]3–n (n = 1–4) were found to be the only form of rhodium(III) existing in the solutions with the metal concentration in the range 0.2–1.3 m. The dynamics of the H2O → NO3– substitution process was studied at 80 °C. An increase in the average number of nitrate groups bonded to rhodium with increasing concentration of nitric acid was also determined. The fine crystalline salt Rb4trans‐[Rh(H2O)2(NO3)4][Rh(NO3)6] was obtained by solvothermal concentration of the rhodium nitric acid solution on addition of rubidium nitrate. The structure of the salt was solved by the powder X‐ray diffraction method, with monodentate coordination of nitrato ligands found for both the [Rh(NO3)6]3– and trans‐[Rh(H2O)2(NO3)4]– anions.