Rate of water exchange between Al(C₂O₄)(H₂O)⁺₄ (aq) complexes and aqueous solutions determined by ¹⁷O-NMR spectroscopy

Substitution of an oxalate molecule for two inner-coordination-sphere waters of Al(H₂O)³⁺₆(aq) enhances, by a factor of ≈10², the rate of exchange of water molecules from the inner-coordination sphere to the bulk solution. The rate parameters for chemical exchange are: k²⁹⁸_ex = 109 s^-1, ΔH‡ = 68.9 ± 2.4 kJ/mol, and ΔS‡ = 25.3 ± 6.7 J/mol/K, measured via dynamic ¹⁷O-NMR. This reactivity enhancement of coordinated waters by oxalate results from a change in bonding between Al(III) and oxygens throughout the complex upon ligation by oxalate. A similar process has been proposed to explain ligand-enhanced dissolution of oxide minerals (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 Ludwig et al. (1995, 1996).