Influence of Countercation Hydration Enthalpies on the Formation of Molecular Complexes: A Thorium–Nitrate Example

Here, the influence of countercations (Aⁿ⁺) in directing the composition of monomeric metal–ligand (ML) complexes that precipitate from solution are often overlooked despite the wide usage of Aⁿ⁺ in materials synthesis. Herein, we describe a correlation between the composition of ML complexes and A⁺ hydration enthalpies found for two related series of thorium (Th)–nitrate molecular compounds obtained by evaporating acidic aqueous Th–nitrate solutions in the presence of A⁺ counterions. Analyses of their chemical composition and solid-state structures demonstrate that A⁺ not only affects the overall solid-state packing of the Th–nitrato complexes but also influences the composition of the Th–nitrato monomeric anions themselves. Trends in composition and structure are found to correlate with A⁺ hydration enthalpies, such that the A⁺ with smaller hydration enthalpies associate with less hydrated and more anionic Th–nitrato complexes. This perspective, broader than the general assumption of size and charge as the dominant influence of Aⁿ⁺, opens a new avenue for the design and synthesis of targeted metal–ligand complexes.