The heteropolyoxoanion-supported complexes [(C(5)Me(5))Rh.P2W15Nb3O62](7-), 1, and [(C6H6)Ru.P2W15Nb3O62](7-), have been prepared by addition of [(C(5)Me(5))Rh(CH3CN)(3)](2+) or [(C6H6)Ru(CH3CN)(3)](2+) to a solution of P2W15Nb3O629- in acetonitrile. Isolation of these complexes as homogeneous, yellow solids as their all-[(n-C4H9)(4)N](+) salts (for 1) or as complexes with mixed [(n-C4H9)4N](+)/Na+ cation composition (for 1 and 2) was accomplished by repeated reprecipitation from acetonitrile with ethyl acetate. Molecular formulas for these complexes were established by complete elemental analyses, in conjunction with a sedimentation-equilibrium molecular-weight measurement. Further characterization in solution relied heavily on multinuclear NMR spectroscopy. The solution data are in accord with [(C(5)Me(5))Rh](2+) and [(C6H6)Ru](2+) being supported on three niobium-bridging oxygens on the "Nb3O93-" surface Of the heteropolyoxoanion. The structural characterization of 1 in the solid state was accomplished by a single-crystal X-ray structural analysis : P6(3)/m; a = 20.544(5), b = 20.544(5), c = 34.648(6) Angstrom; Z = 2; R = 0.096 for 3023 observed independent reflections. The Rh atom in 1 lies on the polyoxoanion’s 3-fold axis within experimental error and is bonded to the three Nb-O-Nb bridging oxygen atoms (Rh-ONb2 2.06 Angstrom) that cap the P2W15Nb3O629- Dawson-type polyoxoanion. This crystallographic analysis, although limited by considerable disorder, represents the first solid-state structure of a Dawson-type heteropolyoxoanion-supported organometallic complex.