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.