First synthesis of group-14 polyanions by extraction of a binary alloy with dmf(1)) and a novel conformation of the (Ge-9-Ge-9)(6-) dimer: Crystal structures of [K-6(Ge-9-Ge-9)](dmf)(12), [Rb-6(Ge-9-Ge-9)](dmf)(12) and [K2.5Cs3.5(Ge-9-Ge-9)](dmf)(12)center dot

Crystals of the title compounds [K-6(Ge-9-Ge-9)](dmf)(12) (1) and [Rb-6(Ge-9-Ge-9)](dmf)(12) (2) have been synthesized by dissolving binary phases of the formal composition K4Ge9 and Rb4Ge9, respectively, in N,N-dimethylformamide (dmf) and subsequent cooling to -36 degrees C. [K2.5Cs3.5(Ge-9-Ge-9)](dmf)(12) (3) was obtained as a by-product during the reaction of K2Cs2Ge9 with sulphur in dmf. Single crystal structure analyses of 1, 2 and 3 show that all compounds contain dimeric [Ge-9-Ge-9](6-) units. The dimers are connected via the alkali metal atoms in form of [(A(+))(3)(mu(2)-dmf)(6)] chains and build a two-dimensional square net of the composition. (2)(infinity){A(6)(dmf)(12)[Ge-9-Ge-9]} with A = K and Rb for 1 and 2, respectively. [(Cs+-K+-Cs+)(mu(2)-dmf)(6)] linkages bridge the dimers in 3 under formation of (2)(infinity){K2.5Cs3.5(dmf)(12)[Ge-9-Ge-9]}. 1 crystallized in the tetragonal space group I (4) over bar with a = b = 34.003(5), c = 17.366(4) angstrom, V = 20078(6) angstrom(3), Z = 8, compound 2 in the triclinic space group P (1) over bar with a = 13.501(3), b = 14.766(3), c = 21.426(4) angstrom, alpha = 88.11(3)degrees, beta = 86.76(3)degrees, gamma = 75.54(3)degrees, V = 4129(1) angstrom(3); Z = 2 and 3 in the monoclinic space group C2/c with a = 21.459(4), b = 17.153(3), c = 22.939(5) angstrom, beta = 93.37(3)degrees, V = 8429(3) angstrom(3), Z = 4. The dimer consists of two Ge-9 clusters, whose shapes are best described as C-2 nu-distorted monocapped square antiprisms. The clusters are connected via an exo Ge-Ge bond with its orientation either co-linear to the shorter diagonal of the open rectangular of the antiprism or radial with respect to the cluster centre in a new type of [Ge-9-Ge-9](6-) dimer.