Kinetic studies of the ligand substitution reaction of some new uranyl schiff base complexes with tri-n-butylphosphine

Article abstract of DOI:10.1002/kin.20815

Kinetics of the substitution reaction of solvent molecule in uranyl(VI) Schiff base complexes by tri-n-butylposphine as the entering nucleophile in acetonitrile at 10-40°C was studied spectrophotometrically. The second-order rate constants for the substitution reaction of the solvent molecule were found to be (8.8 ± 0.5) × 10^-3, (5.3 ± 0.2) × 10^-3, (7.5 ± 0.3) × 10^-3, (6.1 ± 0.3) × 10^-3, (13.5 ± 1.6) × 10^-3, (13.2 ± 0.9) × 10^-3, (52.9 ± 0.2) × 10 ^-3, and (88.1 ± 0.6) × 10^-3 M^-1 s^-1 at 40°C for [UO₂(Schiff base)(CH₃CN)], where Schiff base = L1-L8, respectively. In a temperature dependence study, the activation parameters ΔH^# and ΔS^# for the reaction of uranyl complexes with PBu₃ were determined. From the linear rate dependence on the concentration of PBu₃, the span of k₂ values and the large negative values of the activation entropy, an associative (A) mechanism is deduced for the solvent substitution. By comparing the second-order rate constants k_2, it was concluded that the steric and the electronic properties of the complexes were important for the rate of the reactions.