RATE OF WATER EXCHANGE BETWEEN AL(C2O4)(H2O)(4)(-SOLUTIONS DETERMINEDBY O-17-NMR SPECTROSCOPY()(AQ) COMPLEXES AND AQUEOUS)

Substitution of an oxalate molecule for two inner-coordination-sphere waters of Al(H2O)(6)(3+)(aq) enhances, by a factor of approximate to 1 0(2), the rate of exchange of water molecules from the inner-coordinat ion sphere to the bulk solution. The rate parameters for chemical exch ange are: k(ex)(298) = 109 s(-1), Delta H double dagger = 68.9 +/- 2.4 kJ/mol, and Delta S double dagger = 25.3 +/- 6.7 J/mol/K, measured vi a dynamic O-17-NMR. This reactivity enhancement of coordinated waters by oxalate results from a change in bonding between Al(III) and oxygen s throughout the complex upon ligation by oxalate. A similar process h as been proposed to explain ligand-enhanced dissolution of oxide miner als (e.g. Stumm, 1991; Casey and Ludwig, 1995; Phillips et al., 1997) where a stable adsorbate increases the flux of metals from a surface. These new rate coefficients for aluminum-oxalate complexes, along with previous work on aluminum-fluoride complexes, show a correlation with the respective equilibrium constants similar to that obtained by Ludw ig et al. (1995, 1996). Copyright (C) 1997 Elsevier Science Ltd.