Kinetic Studies of the Ligand Substitution Reaction of Some New Uranyl Schiff Base Complexes with Tri-n-butylphosphine

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 degrees 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)x 10(-3), (5.3 +/- 0.2)x 10(-3), (7.5 +/- 0.3)x 10(-3), (6.1 +/- 0.3)x 10(-3), (13.5 +/- 1.6)x 10(-3), (13.2 +/- 0.9)x 10(-3), (52.9 +/- 0.2)x 10(-3), and (88.1 +/- 0.6)x 10(-3) M-1 s(-1) at 40 degrees C for [UO2(Schiff base)(CH3CN)], where Schiff base= L1-L8, respectively. In a temperature dependence study, the activation parameters H-# and S-# for the reaction of uranyl complexes with PBu3 were determined. From the linear rate dependence on the concentration of PBu3, the span of k(2) 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.