The novel dimeric silicotungstates [{SiM2W9O34(H2O)}(2)](12-) (M = Mn2+, Cu2+, Zn2+) have been synthesized and characterized by IR spectroscopy, elemental analysis, and magnetic measurements. X-ray single-crystal analyses were carried out on K4Na6Mn[{SiMn2W9O34(H2O)}(2)]. 33H(2)O (1), which crystallizes in the triclinic system, space group P (1) over bar, with a = 12.2376(7) Angstrom, b = 13.6764(8) Angstrom, c = 15.6177(9) Angstrom, alpha = 70.2860(10)degrees, beta = 79.9150(10)degrees, gamma = 70.2760(10)degrees, and Z = 1, K3Na5[{SiCu2W9O34(H2O)}(2)]. 26H(2)O (2) crystallizes in the triclinic system, space group P (1) over bar, with a = 11.4271(12) Angstrom, b = 12.5956(13) Angstrom, c = 15.3223(16) Angstrom, alpha = 80.456(2)degrees beta = 76.383(2)degrees, gamma = 76.968(2)degrees and Z = 1; K4Na6[{SiZn2W9O34(H2O)}(2)]. 34H(2)O (3) crystallizes also in the triclinic system, space group P (1) over bar, with a = 12.2596(14) Angstrom, b = 13.2555(15) Angstrom, c = 16.2892(18) Angstrom, alpha = 96.431(2)degrees, beta = 100.944(2)degrees, gamma = 110.404(2)degrees, and Z = 1. The polyanions consist of two lacunary B-alpha-[SiW9O34](10-) Keggin moieties linked via a rhomblike M4O16 (M = Mn, Cu, Zn) group leading to a sandwich-type structure. Magnetic measurements show that the central Mn-4 unit in 1 exhibits antiferromagnetic (J = -1.77(5) cm(-1)) as well as weak ferromagnetic (J' = 0.08(2) cm(-1)) Mn-Mn exchange interactions. In 2 the Cu-Cu exchange interactions are antiferromagnetic (J = -0.10(2) cm(-1), J' = -0.29(2) cm(-1)).