Acidification of an aqueous solution of K8SiW11O39 and K2Pt(OH)6 to pH 4 followed by addition of excess tetramethylammonium (TMA) chloride yielded a solid mixture of TMA salts of H2SiPtW11O40(4-) (1) and SiW12O40(4-) (2). The former was separated from the latter by extraction into an aqueous solution and converted into tetra-n-butylammonium (TBA) and potassium salts TBA-1 and K-1. The α-H2SiPtW11O40(4-) was identified as a monosubstituted Keggin anion using elemental analysis, IR spectroscopy, X-ray crystallography, electrospray ionization mass spectrometry, (195)Pt NMR spectroscopy, (183)W NMR spectroscopy, and (183)W-(183)W 2D INADEQUATE NMR spectroscopy. Both TBA-1 and K-1 readily cocrystallized with their unsubstituted Keggin anion salts, TBA-2 and K-2, respectively, providing an explanation for the historical difficulty of isolating certain platinum-substituted heteropolyanions in pure form.

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