The complex [gamma(1,2)-SiW10{Fe(OH2)}(2)O-38](6-) (1) has been reported to catalyze the much sought reductant-free selective O(2-)based epoxidation of alkenes (Nishiyama, Y.; Nakagawa, Y.; Mizuno, N. Angew. Chem. Int. Ed. 2001, 40, 3639- 3641) in chlorocarbon-acetonitrile solution. The challenge of reproducing catalysis by 1 led us to examine this chemistry in detail. In H2O, a desirable solvent for catalysis, 1, does not exist in the proposed organic-medium form in which the two iron atoms are in the binding pocket defined by the equatorial oxygens and, importantly, by two oxygens bound to the central Si heteroatom. Instead, 1 in H2O initially forms an unusual trimer [{Fe-2(OH)(3)(H2O)(2)}(3)(gamma-SiW10O36)(3)](15-) (2). The X-ray structure of 2 shows that the Fe-O-Si bonds are cleaved and new bonds (A-hydroxo bridges) form between these Fe centers and those of the neighboring [gamma(1,2)-SiW10Fe2] units. Structural, physical, and computational evidence indicate that if the bonds between the d-electron center, M ( Fe in the case of 1 and 2), and the terminal ligands on M are stronger than the M-O-x bonds, then the out-of-pocket form is more stable and is the one observed. Significantly, 2 in H2O forms an intermediate that catalyzes the effective aerobic oxidation of sulfur compounds (mercaptoethanol is oxidized to the corresponding disulfide by O-2 at ambient pressure and temperature). All experimental findings are consistent with dissociation of a gamma-SiW10 Keggin unit from the trimer, 2, to form the catalytically active species.