An ¹⁷O-NMR Study of the Exchange of Water on AlOH(H₂O)₅²⁺(aq)

The pH-dependence of the rate of water exchange between hydrated aluminum complexes and bulk aqueous solution is reported from variable-temperature dynamic ¹⁷O-NMR measurements. Over the temperature range 298-348 K. the pseudo-first-order rate coefficient for exchange at any given temperature, k_ex,obs^T , is described by k_ex,obs^T = k₁^T + k_T²[H⁺]^-1where is the rate coefficient for exchange of inner-sphere water molecules with bulk solution for the Al(H₂O)₆³⁺(aq) complex at that temperature, and k₂^T[H⁺]^-1 is the pH-dependent contribution to k_ex,obs^T from the first hydrolysis product: AlOH(H₂O)₅²⁺(aq). The rate parameters are k₁²⁹⁸ = 1.3 s^-1, ΔH₁‡ = 84/7 kJ·mor^-1, ΔS₁† = 41.6 J·mol^-1·K^-1 (from 6); and k₂²⁹⁸ = 7.2 × 10^-2 = 90.4 kJ·mol^-1, 2*= 6 6 J-mol"1*"1 (this work). From this result we obtained the activation parameters for water exchange on the AlOH(H₂)₅²⁺(aq) complex: k₂₉₈^{ex.AlOH(H2O)52+} = 3.1 × 10⁴= 36.4 kJ·mo1^-1 , ΔS‡ = -36.4 J.mol^-1·^-1 Thus, deprotonation of an inner-sphere water in Al(H₂O)₆³⁺(aq) enhances by a factor of %104 the lability ot other inner-sphere oxygens. This labilizing effect of hydroxyl is much larger than for other geochemically important ligands, including fluoride.