Oxidation of Platinum(II) Nitrate Complexes in Nitric Acid Solutions

The speciation and stability of platinum(II) in concentrated nitric acid solutions are studied, starting from the molecular precursor [Pt(H₂O)₂(OH)₂]. Contrary to the commercially available so-called "platinum(II) nitrate", this research demonstrates the instability of platinum(II) nitrate complexes in nitric acid at concentrations above 5 M. Their oxidation process proceeds through the formation of paramagnetic mixed-valence Pt(II)-Pt(III) binuclear complexes, detected by EPR and ultraviolet-visible (UV-vis) spectroscopy, which rapidly convert to platinum(IV) species. The final products in aged solutions were identified as platinum(IV) nitro-nitrato complexes, rather than simple Pt(II) nitrates. This fact was definitely evidenced by multinuclear NMR spectroscopy (¹⁹⁵Pt and ¹⁵N), which revealed signals that were attributed to Pt(IV) with coordinated nitrite ligands. Furthermore, the binuclear complex [Pt₂(μ-OH)₂(NO₃)₆(NO₂)₂]^2- was successfully isolated in the solid state as tetraalkylammonium salts, confirming the prevalence of oxidized and hydrolyzed species. These findings indicate that attributing commercial platinum nitrate to an oxidation state of +2 is likely erroneous and that stable compounds produced from such solutions are platinum(IV) nitro-nitrato complexes, which are promising precursors for heterogeneous catalysts.