Kinetic Studies of Substitution Reactions of Uranyl(VI) Schiff Base Complexes with Tributylphosphine

The kinetics of the substitution reaction of uranyl Schiff base complexes with tributylphosphine was studied spectrophotometrically in acetonitrile. Uranyl complexes have a pentagonal bipyramidal structure with a trans‐UO₂ moiety at the axial positions. In uranyl tetradentate Schiff base complexes, the fifth position of the equatorial plane is occupied by the solvent molecule, which weakly coordinates to the U center. In a substitution reaction, tributylphosphine can easily replace the solvent molecule. By considering the excellent linearity of k_obs versus the molar concentration of tributylphosphine, the large negative values of Δ S^#, and the small values of Δ H^#, an associative (A) mechanism has been suggested. By comparing the rate constants (k₂) and the activation parameters, it is obvious that two parameters are effective in the rate of substitution reactions; The first parameter is the steric effect that the rate of reaction has been decreased by increasing this factor, and the other parameter is the electronic property that the electron‐withdrawing group leads to increase the rate of reaction and the electron donor group decreases it. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 168–174, 2013