EXPERIMENTAL AND THEORETICAL TREATMENT OF ELEMENTARY LIGAND-EXCHANGE REACTIONS IN ALUMINUM COMPLEXES

Substitution of a hydroxide or fluoride ion for a water molecule in th e inner-coordination sphere of Al(OH2)(6)(3+) considerably weakens bon ds from aluminum to other water molecules that are also in the inner-c oordination sphere. The labilizing effect of these substitutions on th e rate of dissociation of Al-O bonds is a model for ligand-promoted di ssolution of aluminum (hydr)oxide minerals. Here measured activation p arameters for ligand exchange are compared with ab initio calculations of the energetics for comparable reactions. Because solvent exchange is an elementary reaction, it is particularly well-suited for such com parisons. The calculations indicate that substitution of hydroxide or fluoride ion into the inner-coordination sphere greatly reduces the en ergy required to remove a water molecule. The calculated and measured activation energies, however, differ significantly. A reasonable inter pretation is that interactions between hydration waters and second-sph ere water molecules, which are usually partly or fully excluded from t he calculations, contribute to the exchange mechanism.