Reaction of alpha-P2W15O5612- and Fe(III) in a saturated NaCl solution produces a trisubstituted Wells-Dawson structure with three low-valent metals, alpha-((FeCl)-Cl-III)(2)((FeOH2)-O-III)P2W15O5911- (1). Dissolution of this species into I M NaBr (Br- noncoordinating) gives the triaquated species alpha-((FeOH2)-O-III)(3)P2W15O599- (2). Ionic strength values of 1 M or greater are necessary to avoid decomposition of 1 or 2 to the conventional sandwich-type complex, alphaalphabetaalpha-(NaOH2)((FeOH2)-O-III)(2)Fe-2(III)- (P2W15O56)(2)(12-) (3). If the pH is greater than 5, a new triferric sandwich, alphaalphabetaalpha-(NaOH2)((FeOH2)-O-III)Fe-2(III)-(P2W15O56)(2)(14-) (4), forms rather than 3. Like the previously reported Wells-Dawson-derived sandwich-type structures with three metals in the central unit ([TM(II)FTheta(2)(III)(P2W15O56)((P2TM2W13O52)-W-II)],(16-) TM = Cu, Co), this complex has a central alpha-junction and a central beta-junction. Thermal studies suggest that 4 is more stable than 3 over a wide range of temperatures and pH values. The intrinsic Jahn-Teller distotion of d-electron-containing metal ions incorporated into the external sites of the central multimetal unit impacts the stoichiometry of their incorporation (with a consequent change in the inter-POM-unit connectivity, where POM = polyoxometalate). Reaction of nondistorting Ni(II) with the diferric lacunary sandwich-type POM alphaalphaalpha-(NaOH2)(2)Fe-2(III)(P2W15O56)(2)(16-) (5) produces alphabetabetaalpha-((NiOH2)-O-II)(2)Fe-2(III)-(P2W15O56)(2)(14-) (6), a Wells-Dawson sandwich-type structure with two Ni(II) and two Fe(III) in the central unit, All structures are characterized by P-31 NMR, IR, UV-vis, magnetic susceptibility, and X-ray crystallography. Complexes 4 and 6 are highly selective and effective catalysts for the H2O2-based epoxidation of alkenes.