The di- and tetranuclear metal sandwich-type silicotungstates Of (gamma-SiW10O36)(2){Zr(H2O)}(2)(mu-OH)(2)]center dot 18H(2)O (Zr2, monoclinic, C2/c (No. 15), p = 25.3315(8) angstrom, b = 22.6699(7) angstrom, c = 18.5533(6) angstrom, beta = 123.9000(12)degrees, V = 8843.3(5) angstrom(3), Z= 4), Cslo[(y-SiW10O36)2[Hf(H2O)}(2) (mu-OH)(2)]center dot 17H(2)O (Hf2, monoclinic, space group C2/c (No. 15), a = 25.3847(16) angstrom, b = 22.6121(14) angstrom, c = 18.8703(11) angstrom, beta = 124.046(3)degrees, V= 8974.9(9) angstrom(3), Z= 4), CSB[(gamma-Siw(10)O(36))(2)(Zr(H2O)(4)(mu-O)(mu-OH)6]-26H(2)O (Zr4, tetragonal, P4(1)2(1)2 (No. 92), a = 12.67370(10) angstrom, c= 61.6213(8) angstrom, V= 9897.78(17) angstrom 3, Z = 4), and CS8[(gamma-SiW10O36)(2){Hf(H2O))(4)(mu(4)-O)(mu-OH)(6)]-23H(2)O (HM, tetragonal, P4(1)2(1)2 (No. 92), a = 12.68130(10) angstrom, c= 61.5483(9) degrees, V= 9897.91(18) angstrom(3), Z= 4) were obtained as single crystals suitable for X-ray crystallographic analyses by the reaction of a dilacunary gamma-Keggin silicotungstate gamma-SiW10O36 with ZrOCl2 center dot 8H(2)O or HfOCl2 center dot 8H(2)O. These dimeric polyoxometalates consisted Of two gamma-SiW10O36- units sandwiching metal-oxygen clusters such as [M-2(mu-OH)(6)](6+) and [M-4(mu-O)(mu-OH)(6)](8+) (M = Zr or Hf). The dinuclear zirconium and hafnium complexes Zr2 and Hf-2 were isostructural. The equatorially placed two metal atoms in Zr2 and Hf-2 were linked by two mu-OH ligands and each metal was bound to four oxygen atoms of two gamma-SiW10O36 units. The tertanuclear zirconium and hafnium complexes Zr4 and HM were isostructural and consisted of the adamantanoid cages with a tetracoordinated oxygen atom in the middle, [M-4(mu-O)(mu-OH)(6)](8+) (M = Zr or Hf). Each metal atom in Zr4 and HM was linked by three mu-OH ligands and bound to two oxygen atoms of the gamma-SiW10O36 unit. The tetra-nuclear zirconium and hafnium complexes showed catalytic activity for the intramolecular cyclization of (+)-citronellal to isopulegols without formation of byproducts resulting from etherification and dehydration. A lacunary silicotungstate gamma-SiW10O34(H2O)(2)](4-). was inactive, and the isomer ratio of isopulegols in the presence of MOCl2 - 8H(2)O (M = Zr or Hf) were much different from that in the presence of tetranuclear complexes, suggesting that the [M-4(mu-O)(mu-OH)(6)](8+) core incorporated into the POM frameworks acts as an active site for the present cyclization. On the other hand, the reaction hardly proceeded in the presence of dinuclear zirconium and hafnium complexes under the same conditions. The much less activity is possibly explained by the steric repulsion from the POM frameworks in the dinuclear complexes.