Extreme differences in oxidation states: Synthesis and structural analysis of the germanide oxometallates A ₁₀[Ge ₉] ₂[WO ₄] as well as A _10+x[Ge ₉] ₂[W _1-xNb _xO ₄] with A = K and Rb containing [Ge ₉] ^4- polyanions

Semitransparent dark-red or ruby-red moisture- and air-sensitive single crystals of A _10+x[Ge ₉] ₂[W _1-xNb _xO ₄] (A = K, Rb; x = 0, 0.35) were obtained by high-temperature solid-state reactions. The crystal structure of the compounds was determined by single-crystal X-ray diffraction experiments. They crystallize in a new structure type (P2 ₁/c, Z = 4) with a = 13.908(1) Å, b = 15.909(1) Å, c = 17.383(1) Å, and α = 90.050(6)° for K _10.35(1)[Ge ₉] ₂[W _0.65(1)Nb _0.35(1)O ₄]; a = 14.361(3) Å, b = 16.356(3) Å, c = 17.839(4) Å, and α = 90.01(3)° for Rb _10.35(1)[Ge ₉] ₂[W _0.65(1)Nb _0.35(1)O ₄]; a = 13.8979(2) Å, b = 15.5390(3) Å, c = 17.4007(3) Å, and α = 90.188(1)° for K ₁₀[Ge ₉] ₂WO ₄; and a = 14.3230(7) Å, b = 15.9060(9) Å, c = 17.8634(9) Å, and α = 90.078(4)° for Rb ₁₀[Ge ₉] ₂WO ₄. The compounds contain discrete Ge ₉ ^4- Wades nido clusters and WO ₄ ^2- (or NbO ₄ ^3-) anions, which are packed according to a hierarchical atom-to-cluster replacement of the Al ₂Cu prototype and are separated by K and Rb cations, respectively. The alkali metal atoms occupy the corresponding tetrahedral sites of the Al ₂Cu prototype. The amount of the alkali metal atoms on these diamagnetic compounds corresponds directly to the amount of W substituted by Nb. Thus, the transition metals W and Nb appear with oxidation numbers +6 and +5, respectively, in the vicinity of a [Ge ₉] ^4- polyanion. The crystals of the mixed salts were further characterized by Raman spectroscopy. The Raman data are in good agreement with the results from the X-ray structural analyses.