A density functional theoretical study on solvated Al3+-oxalate complexes:structures and thermodynamic properties

Density functional theory calculations using split-valence polarized basis sets augmented by diffuse s and p functions on the carbon and oxygen atoms were carried out for the complexes of hydrated Al3+ with oxalic acid, and t he oxalate mono- and dianions. Monodentate and bidentate structures with up to three ligands have been computed. The polarized continuum model was use d to study the solvent effect on the structures and stabilities of the comp lexes. Reaction energies for the replacement of water molecules in the alum inum-hexaaquo complex by oxalate ligands have been computed. Based on a det ailed thermodynamical analysis, characteristic differences in the formation of mono- and bidentate structures were found. In the former case the entro py contributions to Delta G are rather small, whereas in the latter case th ey are substantial. Thus, the formation reactions for the monodentate compl exes are energy-driven whereas those for the bidentate complexes are entrop y-driven. In agreement with experiment, the most dominant complex in soluti on is [AlOx(3)](3-). From the complexes with oxalic acid and the partially deprotonated form HOx(-), only the latter should give stable bidentate comp lexes in solution.