When Counterions Fall into Place: Alkali Effects on f-Element Polyoxomolybdate Crystallization and Speciation

Speciation and crystallization of f-elements with polyoxomolybdate (POMo) ligands have been scarcely studied compared to analogous tungstate POMs. Here we investigate polyoxomolybdate complexes with trivalent lanthanides (LnIII) in solution and the solid state to detail the interplay between the f-elements, POM ligand, and counterions. Alkali counterions play a decisive role in shaping the solid-state structure and solution-state behavior of POM complexes, including the Ln-POMo reported here. A family of Ln(PMo11O39)211– compounds with charge-balancing K+ and Cs+ counterions were synthesized across the lanthanide series, with 16 total structures determined by single-crystal X-ray diffraction, complemented by bulk solid characterization. Combined solid-state and solution-state studies enabled direct comparison of structural trends and revealed a countercation-dependent solution-state speciation. While K+ analogues exhibit consistent parallel arrangement of POM units in the crystalline lattice, Cs+ derivatives adopt distinct packing modes, including both parallel and perpendicular orientations. These structural differences are reflected in various lattice organizations and intercluster interactions, highlighting the role of larger alkali cations in directing long-range assembly. Moreover, solution-state 31P nuclear magnetic resonance (NMR) spectroscopy and small-angle X-ray scattering (SAXS) reveal that the countercation (Li+, Na+, K+, Rb+, and Cs+ studied) directly impacts assembly and dynamic equilibria among the aqueous species PMo11O397–, Ln(PMo11O39)4– and Ln(PMo11O39)211–, prior to crystallization.