The dimer alpha-[{K(H2O)(2)}(Si2W18O66)](15-) (1), synthesized by reacting K(10)A-alpha-[SiW9O34] with two equivalents of H+ in aqueous solution, has been characterized by polarography and W-183 NMR spectroscopy. Nine resonance signals have been observed with the tetrabutylammonium salt in dimethylformamide/acetonitrile solution, in agreement with the crystal structure of the anion which consists of two A-alpha-[SiW9O34](10-) associated through two W-O-W junctions. This anion derives from the Wells-Dawson structure by breaking four W-O-W junctions. The pocket between the two-half-anions can be filled by several metal cations. Reaction of transition-metal cations with 1 leads to the formation of [{M(H2O)}(mu-H2O)(2)K(Si2W18O66)](13-) (1M) (M = Co, Ni, Cu) and [{M(H2O)}(mu-H2O)(2)K{M(H2O)(4)}(Si2W18O66)(11-) (1M2) (M = Mn, Co, Ni) complexes. One potassium is always included in the pocket with one or two transition metals. Because of the shift of the potassium cation to one side of the anion, the coordination modes of the two transition metals are different. Crystals of 1, 1M, and 1Co2 potassium salts are triclinic (P-1, Z = 2) and crystals of 1M2 potassium salts are monoclinic (P2(1)/n, Z = 4). The symmetry of 1M and 1M2 complexes is C1 and they are present in the crystal as racemate inversion pairs.