Complexation of trinuclear oxo-centered carboxylates with a silicododecatungstate resulted in the formation of ionic crystals of [M3O - (OOCC6H5)(6)(H2O)(3)](4)[alpha-SiW12O40]center dot nH(2)O center dot mCH(3)C-OCH3 [M = Cr (Ia), Fe (IIa )]. Treatments of Ia and IIa at 373 K in vacuo formed guest-free phases Ib and IIb, respectively. Compounds Ib and IIb heterogeneously catalyzed the pinacol rearrangement to pinacolone with high conversion at 373 K, and the catalysis is suggested to proceed size selectively in the solid bulk.