Kinetics and Equilibrium of the Reaction between Bis(trifluoroacetylacetonato)palladium(II) and Tri-o-tolylphosphine

Tri-o-tolylphosphine reacts with bis(trifluoroacetylacetonato)palladium(II), Pd(tfac)2, to convert one of the chelating ligands into the unidentate state, resulting in Pd(tfac)(tfac-O)P(o-tolyl)3. The reaction was studied by a spectrophotometric method. The equilibrium constant was determined at several temperatures, K being 1.38×103, 4.35×103 dm3 mol−1 and more than 109 dm3 mol−1 in benzene, dichloromethane, and methanol, respectively, at 25 °C. In the presence of excess phosphine, the reaction proceeds as a pseudo first order reaction to attain equilibrium. The forward and reverse rate constants, k1 and k−1, were obtained as 0.172 dm3 mol−1 s−1 and 1.22×10−4 s−1 in benzene and 2.28 dm3 mol−1 s−1 and 4.9×10−4 s−1 in dichloromethane at 25 °C, respectively. In methanol the forward rate was measured under irreversible pseudo first order conditions and conforms with the usual two term rate law, kobsd⁄s−1=2×10−3+56.6[P(o-tolyl)3]. The remarkable dependence of K and k1 on the nature of solvent is related to the hydrogen bonding interaction between the carbonyl group of the coordinated tfac anion and a solvent molecule.