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

Effects controlling the speciation and crystallization of f-elements with polyoxomolybdate (POMo) ligands have been scarcely studied compared to the analogous tungstate POMs. Here we investigate polyoxomolybdate complexes with trivalent lanthanides (Ln 3+ ), in solution and solid-state, shedding lights on the complex interplay between the f-elements, POM ligand, and counterions. Alkali counterions play a decisive role in shaping the structure and behavior of POM complexes, including the lanthanide-POMo complexes reported here. A family of Ln(PMo 11 O 39 ) 2 11- compounds with charge-balancing K + and Cs + were synthesized across the lanthanide series, and 16 total structures were characterized by SCXRD, FTIR, and SEM-EDX. Combined solid-state and solution-phase studies enabled direct comparison of structural trends and countercation-dependent solution-state speciation. While potassium analogues exhibit consistent parallel arrangement of POM units in the crystalline lattice, cesium derivatives adopt distinct packing modes, including both parallel and perpendicular orientations. These structural differences are reflected in variations in lattice organization and inter-cluster interactions, highlighting the role of larger alkali cations in directing long-range assembly. Moreover, solution-state 31 P 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 thermodynamic equilibria among PMo 11 O 39 7- , Ln(PMo 11 O 39 ) 4- and Ln(PMo 11 O 39 ) 2 11- , prior to crystallization.