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Inorganic salts usually demonstrate simple phasal behaviors in dilute aqueous solution mainly involving soluble (homogeneous) and insoluble (macrophase separation) scenarios. Herein, we report the discovery of complex phase behavior involving multiple phase transitions of clear solution – macrophase separation – gelation – solution – macrophase separation in the dilute aqueous solutions of a structurally well-defined molecular cluster [Mo₇O₂₄]⁶⁻macroanions with the continuous addition of Fe³⁺. No chemical reaction was involved. The transitions are closely related to the strong electrostatic interaction between [Mo₇O₂₄]⁶⁻and their Fe³⁺counterions, the counterion-mediated attraction and the consequent charge inversion, leading to the formation of linear/branched supramolecular structures, as confirmed by experimental results and molecular dynamics simulations. The rich phase behavior demonstrated by the inorganic cluster [Mo₇O₂₄]⁶⁻expands our understanding of nanoscale ions in solution.

The critical gelation conditions observed in dilute aqueous solutions of multiple nanoscale uranyl peroxide molecular clusters are reported, in the presence of multivalent cations. This gelation is dominantly driven by counterion‐mediated attraction. The gelation areas in the corresponding phase diagrams all appear in similar locations, with a characteristic triangle shape outlining three critical boundary conditions, corresponding to the critical cluster concentration, cation/cluster ratio, and the degree of counterion association with increasing cluster concentration. These interesting phrasal observations reveal general conditions for gelation driven by electrostatic interactions in hydrophilic macroionic solutions.

A unique trend in the binding affinity between cationic metal−organic cages (MOCs) and external counteranions in aqueous media was observed. Similar to many macroions, two MOCs, sharing similar structures but carrying different number of charges, self‐assembled into hollow spherical single‐layered blackberry‐type structures through counterion‐mediated attraction. Dynamic and static light scattering and isothermal titration calorimetry measurements confirm the stronger interactions among less charged MOCs and counteranions than that of highly charged MOCs, leading to larger assembly sizes. DOSY NMR measurements suggest the significance of thick hydration shells of highly charged MOCs, inhibiting the MOC‐counterion binding and weakening the interaction between them. This study demonstrates that the greater role played by hydration shell on ion‐pair formation comparing with charge density of MOCs.