A density-functional investigation of aluminium(III)-citrate complexes

Density-functional calculations have been performed on tridentate, hydrated aluminium-citrate complexes using fully (quadruply) and triply deprotonate d citric acid ligands. Water molecules in the inner solvation sphere have b een included explicitly in the quantum-chemical calculation, whereas the re maining solvent effects have been computed using the polarized continuum mo del (PCM). As is to be expected, solvation effects play an important role f or the calculation of formation energies of the complexes. Optimized geomet ries are in good agreement with X-ray data. Reaction enthalpies and Gibbs r eaction energies have been computed for the substitution of water molecules of the aluminium-hexaaquo complex by citrate molecules. Formation of the t ridentate complexes is strongly favored by entropy effects in analogy to pr evious findings for bidentate acetate and oxalate complexes. Comparison of the stability of acetate, oxalate and citrate complexes shows a pronounced preference for the latter.