Rhodium(III) Speciation in Concentrated Nitric Acid Solutions

The interaction of rhodium(III) aqua ions with nitrate ions in 3–16 m nitric acid solutions has been studied by¹⁰³Rh and¹⁵N 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.