Stabilization of β-octamolybdate with large counterions

Two new organic-inorganic compounds [H2NC(N(CH3)2)2]4[Mo8O26]·2H2O (1) and (C6H11NH3)4[Mo8O26] (2) have been synthesized at room temperature and characterized by single-cristal X-ray diffraction, thermal and elemental analyses. We show that the β-[Mo8O26]4− isomer can be stabilized by counterions which are either small or large. This observation is contradictory to the common rule which states that the β isomer should be stabilized by small counterions. Thus, we show that the organic counterion in both compounds (1) and (2) are large, but its ammonium group stabilize the β form owing to its localization in a specific nucleophilic site. The octamolybdate β-[Mo8O26]4− is shown to be stabilized if an electrophile (small or large) can be placed between four terminal oxides. [Display omitted]