We report the synthesis, spectroscopic and structural characterization, and computational analysis of a series of phosphomolybdate complexes with tetravalent metal cations. The reaction between Ce-IV and Th-IV with phosphomolybdate at the optimum pH for the stabilization of the lacunary heteropolyoxometalate anion, [PMo11O39](7-), results in the formation of compounds containing the anions [Ce(PMo11O39)(2)](10-) and [Th(PMo11O39)(2)](10-), respectively. Single crystal X-ray diffraction analysis was performed on salts of both species, Cs-10[Ce(PMo11O39)(2)]center dot 20H(2)O and (NH4)(10)P(PMo11O39)(2)]center dot 22H(2)O. In both anionic complexes the f-block metal cation is coordinated to the four unsaturated terminal lacunary site oxygens of each [PMo11O39](7-) anion, yielding 8 coordinate sandwich complexes, analogous to previously prepared related complexes. Spectroscopic characterization points to the stability of these complexes in solution over a reasonably Wide pH range, Density functional analysis suggests that the Ce-O bond strength in [Ce(PMo11O39)(2)](10-) is greater than the Th-O bond strength in [Th(PMo11O39)(2)](10-), with the dominant bonding interaction being ionic in both cases. In contrast, under similar reaction conditions, the dominant solid state Zr-IV and Hf-IV complexes formed contain the anions [Zr(PMo12O40)(PMo11O39)](6-) and [Hf(PMo12O40)(PMo11O39)](6-), respectively. In these complexes the central Group 4 d-block metal cations are coordinated to the four unsaturated terminal lacunary site oxygens of the [PMo11O39](7-) ligand and to four bridging oxygens of a plenary Keggin anion, [PMo12O40](3-). In addition, (NH4)(5){Hf[PMo12O40][(NH4)PMo11O39]}-23.5H(2)O can be crystallized as a minor product. The structure of the anion, {Hf[PMo12O40][(NH4)PMo11O39]}(5-), reveals coordination of the central Hf-IV cation via four bridging oxygens on both the coordinated [PMo11O39](7-) and [PMo12O40](3-) anions. Unusually, the highly charged lacunary site remains uncoordinated to the Hf metal center but instead interacts with an ammonium cation, P-31 NMR indicates that complexation of the Keggin anion, [PMo12O40](3-), to Hf-IV and Zr-IV will stabilize the Keggin anion to a much higher pH than usually observed. ; Copping, Roy Jonasson, Leif Gaunt, Andrew J. Drennan, Dennis Collison, David Helliwell, Madeleine Pirttijarvi, Ross J. Jones, Chris J. Huguett, Anne Apperley, David C. Kaltsoyannis, Nikolas May, Iain 48 AMER CHEMICAL SOC WASHINGTON 321BB