The reaction of the triniobium-substituted polyoxometalate [(n-C4H9)(4)N]9P2W15Nb3O62 with an equimolar amount of [Ir(1,5-COD)(CH3CN)(2)]BF4 or [Rh(1,5-COD)(CH3CN)(2)]BF4 (1,5-COD = 1,5-cyclooctadiene) leads to the formation of the air-sensitive polyoxometalate-supported organometallic complexes [(1,5-COD)IrP2W15Nb3O62](8-), and [(1,5-COD)Rh . P2W15Nb3O62](8-), 2. These complexes were isolated as their mixed 5[(n-C4H9)(4)N](+)/3Na(+) salts and have been characterized by H-1, C-13, P-31, and W-183 NMR spectroscopy as well as IR spectroscopy, sedimentation-equilibrium molecular-weight measurements, and complete elemental analyses. Additional studies of 1 by O-17 NMR demonstrate that the iridium binds in overall average C-3 nu (pseudo) symmetry to the "Nb3O93-" minisurface (pseudo due to the 2-fold axis in 1,5-COD and thus the local C, symmetry at iridium). For 2, the results of the O-17 NMR studies are definitive in showing that 2 can also be successfully O-17-enriched in the niobium-oxygen sites. However, the O-17 NMR data subsequently acquired for 2 require the formulation of two or more (possibly rapidly interconverting) support-site isomers in solution. These O-17 NMR results provide direct evidence for the M-ONb2 bonding between [(1,5-COD)M](+) (M = Ir, Ph) and P2W15Nb3O629- in solution, where catalysis beginning with 1 and 2 as a precatalyst has been demonstrated.