Mixed-metal sandwich complexes [M-2(II)(H2O)(2)Fe-2(III)(P2W15O56)(2)](14-) (M-II = Co, Mn): Synthesis and stability. The molecular structure of [M-2(II)(H2O)(2)Fe-2(III)(P2W15O56)(2)](14-)

Two new mixed-metal sandwich complexes [M-2(II)(H2O)(2)Fe-2(III)(P2W15O56)(2)](14-) (abbreviated {M2Fe2P4W30}, M-II = Co-II, Mn-II) were obtained at pH 3 by addition of M2+ to [Na-2(H2O)(2)Fe-2(III)(P2W15O56)(2)](16-) (abbreviated {Na2Fe2P4W30}) without substitution in the alpha-[P2W15O56](12-) (abbreviated {P2W15}) units. Their X-ray structures are reported. At lower pH, back conversion to {Na2Fe2P4W30} was followed by P-31 NMR, electrochemistry and UV-visible spectroscopy. The preparation and the characterization in solution of the lacunary intermediate [NaCoII(H2O)(2)Fe-2(III)(P2W15O56)(2)](15-) (abbreviated {NaCoFe2P4W30}) is also described.