Distinctive Interactions of Cesium and Hexaniobate in Water

The Cs-effect states Cs⁺ has more covalent character in bonding interactions than the lighter alkalis. It is exploited in organic synthesis and influences behavior in water, most notably radioactive ¹³⁷Cs in nuclear wastes or the environment. Niobium polyoxometalates (Nb-POMs) provide a unique opportunity to probe aqueous phase ion-pairing responsible for cesium's solution behavior, because Nb-POMs are most soluble in conditions of maximum ion-association. Moreover, POMs broadly resemble metal-oxide surfaces representative of interfaces found in the environment and industrial processes. Aqueous dissolution calorimetry reveals that Cs−Nb-POM exhibits greater concentration dependence in its endothermic dissolution, compared to the lighter alkali analogues. This phenomenon is attributed to persistent ion-pairs upon dissolution, even in very dilute and otherwise ion-free solutions. While dissociation of these cation-anion interactions in the crystalline lattice is the dominant endothermic step of dissolution, deprotonation of the Nb-POM is the most exothermic. These studies highlight the importance of the competing effects of aqueous ion association and acid-base chemistry that control solubility of compounds from simple oxoanions to metal-oxo clusters to supramolecular assemblies to solid metal oxides.