The syntheses and crystal structures of a series of zirconium(IV) and hafnium(IV) complexes with Dawson monovacant phosphotungstate [alpha(2)-P2W17O61](10-) and in situ-generated Keggin monovacant phosphotungstate [alpha-PW11O39](7-), which was obtained by a reaction of [alpha-PW12O40](3)-with Na2CO3, are described. K15H[Zr(alpha(2)-P2W17O61)(2)]center dot 25H(2)O (K-1), K-16[Hf(alpha(2)-P2W17O61)(2)]center dot 19H(2)O (K-2), (t(2)NH(2))(10)[Zr(alpha-PW11O39)(2)]center dot 7H(2)O (Et2NH2-3), and (Et2NH2)(10)[Hf(alpha-PW11O39) (2)]center dot 2H(2)O (Et2NH2-4), being afforded by reactions in aqueous solutions of monolacunary Dawson and Keggin polyoxotungstates with ZrCl2O center dot 8H(2)O and HfCl2O center dot 8H(2)O followed by exchanging countercations, were obtained as analytically pure, homogeneous colorless crystals. Single-crystal X-ray structure analyses revealed that the Zr(IV) and Hf(IV) ions are in a square antiprismatic coordination environment with eight oxygen atoms, four of them being provided from each of the two monovacant polyanion ligands. Although the total molecular shapes and the 8-coordinate zirconium and hafnium centers of complexes 1 -4 are identical, the bonding modes (bond lengths and bond angles) around the zirconium(IV) and hafnium(IV) centers were dependent on the monovacant structures of the polyanion ligands. Additionally, the characterization of complexes 1-4 was accomplished by elemental analysis, TG/ DTA, FTIR, and solution (P-31 and W-183) NMR spectroscopy.