Reaction of the diferric sandwich-type polyoxometalate (NaOH2)(2)Fe-2(III)(P2W15O56)(2)(16-) (1) with excess aqueous Cu(II) or Co(II) yields a new type of d-electron-metal substituted polyoxometalate, [(TMFe2III)-Fe-II(P2W15O56) ((P2TM2W13O52)-W-II)](16-), TM = Cu (2), Co (3), respectively. The structure of the sodium salt of 2 (Na2), determined by single-crystal X-ray diffraction analysis (a = 13.44,13(9) Angstrom, b = 21.2590(15) Angstrom, c = 25.5207(18) Angstrom, alpha = 80.475(2)degrees, beta = 85.555(2)degrees, gamma = 89.563(2)degrees, triclinic, P (1) over bar, R1 = 5.42%, based on 43097 independent reflections), consists of a defect Fe2Cu central unit sandwiched between two different trivacant Wells-Dawson-type units, P2W15 and P2Cu2W13, where the latter unit has two octahedral Cu(II) ions substituted for two adjacent belt W(VI) atoms. The CuO5OH2 octahedron in the central unit shows pronounced Jahn-Teller distortion. A low-resolution X-ray structure of Na3 is included in the Supporting Information. UV-visible, infrared, P-31 NMR, cyclic voltammetric, and elemental analysis data are all consistent with the structure determined from the X-ray analysis. Cyclic voltammograms of 2 and 3 exhibit multiple electron-transfer processes under ambient conditions, and copper or cobalt incorporation into the framework of I results in a substantial pertubation of the electrochemical properties of the polyoxotungstate framework. The tetra-ii-butylammonium salts of 2 and 3 (readily prepared by metathesis) are stable and effective catalysts for the oxidation of some alkenes with high yields based on H2O2.