Experimental and theoretical treatment of elementary ligand exchange reactions in aluminum complexes

Substitution of a hydroxide or fluoride ion for a water molecule in the inner-coordination sphere of Al(OH₂)₆³⁺ considerably weakens bonds from aluminum to other water molecules that are also in the inner-coordination sphere. The labilizing effect of these substitutions on the rate of dissociation of Al-O bonds is a model for ligand-promoted dissolution of aluminum (hydr)oxide minerals. Here measured activation parameters 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 comparisons. The calculations indicate that substitution of hydroxide or fluoride ion into the inner-coordination sphere greatly reduces the energy required to remove a water molecule. The calculated and measured activation energies, however, differ significantly. A reasonable interpretation is that interactions between hydration waters and second-sphere water molecules, which are usually partly or fully excluded from the calculations, contribute to the exchange mechanism.