Density functional modeling of ternary Am(III) hydroxo complexes with Ca or Mg counterions. Do Mg stabilized species exist?

We carried out density functional computations to investigate experimentally suggested ternary Ca–Am(III)–OH and possible Mg–Am(III)–OH complexes in water. We confirmed the experimental stoichiometry for the Ca complexes and their relative stability. For the Mg complexes, we find comparable or weaker complexation. This finding is in agreement with experimental observations. We demonstrate that explicit solvation of counterions is important when comparing Ca and Mg species. For the Ca complexes this has a minimal effect on their relative stability. Contrariwise, Mg complexes exhibit a lower stability and are highly sensitive to explicit short-range solvation effects. Explicit Mg 2+ solvation significantly altered both absolute and relative formation energies compared to a simpler model. These findings highlight the crucial role of incorporating explicit short-range solvation effects in accurate computational modeling when small and highly charged ions are treated. • Confirmed: Structure and stability of Ca–Am(III)–OH complexes in alkaline conditions. • Mg–Am(III)–OH complexes show lower or similar stability, minor role in speciation. • Explicit Ca ion solvation has little impact; Mg solvation strongly affects stability. • Explicit short-range solvation is crucial for small, highly charged cations.