KINETICS OF THE SUBSTITUTION OF THE CYCLOOCTADIENE LIGAND IN BETA-DIKETONATOCYCLOOCTADIENERHODIUM(I) COMPLEXES BY TRIPHENYLPHOSPHITE

The reaction between [Rh(β-diketonato)(COD)] complexes and triphenylphosphite was studied for various β-diketones in a dichloroethane medium. The observed rate law is R = k[Rh(β-diketonato)(COD)][P(OPh)3]. The reaction rate increases in the order acac < BA < DBM ≪ TFAA < TFBA ≪ HFAA (acac = acetylacetone, BA = benzoylacetone, DBM = dibenzoylacetone, TFAA = 1,1,1- trifluoro-2,4-pentanedione, TFBA = 1,1,1-trifluoro-5-phenyl-2,4-pentanedione, HFAA = 1,1,1-trifluoro-5,5,5-trifluoro-2,4-pentanedione), indicating that electro-negative substituents of the β-diketone increase the reactivity of these complexes towards substitution reactions. A good linear free energy relationship was obtained. The large negative values for the entropy of activation, as well as the order of reactivity of the different β-diketone complexes, point towards an associative mechanism. © 1990.