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.