Synthesis, Crystal Structure, and Solution Stability of Keggin-Type Heteropolytungstates (NH4)6NiII 0.5[α-FeIIIO4W11O30NiIIO5(OH2)]·nH2O, (NH4)7Zn0.5[α-ZnO4W11O30ZnO5(OH2)]·nH2O, and (NH4)7NiII 0.5[α-ZnO4W11O30NiIIO5(OH2)]·nH2O (n ≈ 18)

Reaction of acidified (pH ≈ 7) sodium tungstate solutions with transition metal cations (Fe3+, Ni2+, Zn2+, Co2+) leads to the formation of transition-metal-disubstituted Keggin-type heteropolytungstates with 3d-metal ions distributed over three different positions. A detailed investigation of the synthesis conditions confirmed that the complexes could equally be obtained using aqueous solutions of either Na2WO4·2H2O (sodium monotungstate) at pH ≈ 7, Na6[W7O24]·∼14H2O (sodium paratungstate A), or Na10[H2W12O42]·27H2O (sodium paratungstate B) as starting materials. Three complexes, (NH4)6NiII 0.5[α-FeIIIO4W11O30NiIIO5(OH2)]·18H2O, (NH4)7Zn0.5[α-ZnO4W11O30 ZnO5(OH2)]·18H2O, and (NH4)7NiII 0.5[α-ZnO4W11O30NiIIO5(OH2)]·18H2O were isolated in crystalline form. X-ray single-crystal structure analysis revealed that the solid-state structures of the three compounds consist of four main structural fragments, namely [MO4W11O30M‘O5(OH2)] n - (Keggin-type, α-isomer) heteropolytungstates, hexaquo metal cations, [M‘ ‘(OH2)6]2+, ammonium−water cluster ions, [(NH4 +)8(OH2)12], and additional ammonium cations and water molecules. The 3d metals occupy the central (tetrahedral, M) and the peripheral (octahedral, M‘) positions of the Keggin anion, as well as cationic sites (M‘ ‘) outside of the polyoxotungstate framework. UV−vis spectroscopy, solution (1H, 183W) and solid-state (1H) NMR, and also chemical analysis data provided evidence that the 3d-metal-disubstituted Keggin anions do not exist in solution but are being formed only during the crystallization process. Investigations in the solid state and in solution were completed by ESR, IR, and Raman measurements.