Speciation of Platinum(IV) in Nitric Acid Solutions

The speciation of platinum­(IV) ions
in nitric acid (6–15.8
M) solutions of H2[Pt­(OH)6] has been studied
by 195Pt NMR and Raman spectroscopy. Series of aqua-hydroxo-nitrato
complexes [Pt­(L)x(NO3)6–x] (L = H2O or OH–; x = 0, ..., 6) were found
to exist in such solutions. The pair additivity model of chemical
shifts and statistical theory were used to assign signals in NMR spectra
to particular [Pt­(L)x(NO3)6–x] species. Mononuclear hexanitratoplatinates­(IV)
have been isolated in solid state in substantial yield as pyridinium
salt (PyH)2[Pt­(NO3)6] and characterized
by single-crystal X-ray diffraction. Aging of the platinum nitric
acid solutions for more than 5–6 h results in oligomerization
of [Pt­(L)x(NO3)6–x] species and the formation of oligonuclear aqua-hydroxo-nitrato
complexes with OH– and NO3– bridging ligands. Oligomeric platinum­(IV) complexes with two and
four nuclei were unambiguously detected by NMR on 195Pt
-enriched samples. Oligomers with even higher nuclearity were also
detected. Dimeric anions [Pt2(μ-OH)2(NO3)8]2– have been isolated as single
crystals of tetramethylammonium salt and characterized by X-ray diffraction.