SOLVENT EXCHANGE IN ALFX(H2O)(6-X)(3-X) (AQ) COMPLEXES - LIGAND-DIRECTED LABILIZATION OF WATER AS AN ANALOG FOR LIGAND-INDUCED DISSOLUTION OF OXIDE MINERALS

We demonstrate, using dynamic O-17-NMR spectroscopy, that fluoride ion s in the inner-coordination sphere of AlFx(H2O)(6-x)(3-x)(aq) complexe s (0 less than or equal to x less than or equal to 2) progressively en hance, 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 fl uoride substitution from about 2 s(-1) for the fully hydrated [Al(H2O) (6)(3+)(aq)] complex to about 2 x 10(4) s(-1) for the difluoro [AlF2(H 2O)(4)(+)(aq)] complex. A similar effect can be expected for isoelectr onic complexes, such as the hydrolysis product: AlOH(H2O)(5)(2+)(aq). By hypothesis, a similar phenomenon accounts for fluoride-enhanced rel ease of metals from a dissolving mineral surface. If so, it should be possible to predict the effectiveness adsorbed ligands to enhance diss olution of minerals from spectroscopic rate measurements on dissolved complexes, as was recently shown for aminocarboxylate ligands and NiO( s) (Ludwig et al., 1995, 1996). Copyright (C) 1997 Elsevier Science Lt d.