Mechanism of ligand substitution in aluminum(III) complexes

Kinetics of Ligand Exchange Reactions for Uranyl(2+) Fluoride Complexes in Aqueous Solution

“…Complex formation reactions between hard donors and acceptors are in general accompanied by large and positive entropy changes, indicating substantial changes in the solvation between reactants and products. The available activation entropies for many reactions of this type are also large and positive, as exemplified by data for the beryllium(II)-, aluminum(III)-, and iron(III) −fluoride systems; Δ S ⧧ = 83 J mol -1 K -1 for the formation of Al(H 2 O) 5 F 2+ , indicating a transition state close to the product. The negative entropy of activation for reaction 21 is unexpected and indicates the formation of an intermediate that does not involve any substantial change of hydration between the reactants and the transition state.…”

DFT study on the mechanism for the substitution of F−into Al(iii) complexes in aqueous solution

“…The previous investigation suggests that the substitution of Finto the inner-coordination sphere of Al(H 2 O) 6 3+ (eqn (1), pH = 3) proceeds via the I a mechanism, 16 but the I d mechanism is also believed to be preferred by some researchers. 3a Here possible substitution mechanisms including the A mechanism (eqn (2)), the I mechanism (eqn (3)) and the D mechanism (eqn (4)) are modeled to determine the most preferred pathway.…”

“…This implies that the I d mechanism is the preferred substitution pathway for eqn (5) and the previous experimental results draw the same conclusion as well. 16 The mechanistic variation from Al(H 2 O) 2+ , where the mechanism changeover from I a to I d occurs with decreasing charge. 24 Recently Rotzinger also reported that the spectator anion ligand causes the water-exchange mechanism of UO 2 2+ changing from associative to dissociative.…”

“…This agrees well with the experimental results that the rates increase by about three orders of magnitude in the presence of the OHligand (See Table 3). 16 This point needs to be considered in studying the fluoride-promoted and proton-promoted dissolution of minerals.…”

“…For the present reactions, the experimental rate constants were obtained by a potentiometry method described previously. 16 Table 3 gives the experimental and computational activation parameters for the two substitution reactions. For the activation enthalpy, the difference between the theoretical and experimental values is significant.…”