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

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.