Solvent exchange in AlF_x(H₂O)^3-x_6-x (aq) complexes: Ligand-directed labilization of water as an analogue for ligand-induced dissolution of oxide minerals

We demonstrate, using dynamic ¹⁷O-NMR spectroscopy, that fluoride ions in the innercoordination sphere of AlF_x(H₂O)^3-x_6-x (aq) complexes (0 ≤ x ≤ 2) progressively enhance, by orders of magnitude, the rate of exchange of waters from the innersphere to the bulk solvent. At 298K, the rate of the elementary ligand-exchange reaction increased approximately linearly with each fluoride substitution from about 2 s^-1 for the fully hydrated [Al(H₂O)³⁺₆ (aq)] complex to about 2 × 10⁴ s^-1 for the difluoro [AlF₂ (H₂O)⁺₄ (aq)] complex. A similar effect can be expected for isoelectronic complexes, such as the hydrolysis product: AlOH(H₂O)²⁺₅ (aq). By hypothesis, a similar phenomenon accounts for fluoride-enhanced release of metals from a dissolving mineral surface. If so, it should be possible to predict the effectiveness of different adsorbed ligands to enhance dissolution of minerals from spectroscopic rate measurements on dissolved complexes, as was recently shown for aminocarboxylate ligands and NiO(s) (Ludwig et al., 1995, 1996).