Counterion Interaction and Association in Metal-Oxide Cluster Macroanionic Solutions and the Consequent Self-Assembly

The interaction between large metal-oxide polyanions and their counterions is unique. Owing to their size disparity, there is a moderate ion-pairing effect and loose distribution of counterions around macroions, which leads to the unique solution behavior and the self-assembly of the macroions in polar solvents, and the counterion exchange capability around macroions. Furthermore, the macroion–counterion interaction also affects the catalytic behavior of the polyoxometalate (POM) clusters. Replacement of functionalized cations helps to modify the POM anions through static charge attraction. At the same time, the strong POM–counterion interaction can also lead to counterion-dependent synthesis. Recent developments on theoretical simulations help to understand this interaction at the molecular scale. This review summarizes the chronological progress of the exploration of macroion–counterion interaction (both theoretically and experimentally) and its impact on related research fields.