Ligand substitution kinetics in M(CO)(4)(eta(2 : 2)-1,5-cyclooctadiene) complexes (M = Cr, Mo, W) - substitution of 1,5-cyclooctadiene by bis(diphenylphosphino)alkanes

The thermal substitution kinetics of 1,5-cyclooctadiene (COD) by bis(diphen ylphosphino)alkanes (PP), (C6H5)(2)P(CH2)(n)P(C6H5)(2) (n = 1, 2, 3) in M(C O)(4)(eta(2:2)-COD) complexes (M = Cr, Mo, W), were studied by quantitative FT-LR spectroscopy. The reaction rate exhibits first-order dependence on t he concentration of the starting complex, and the observed rate constant de pends on the concentration of the leaving COD ligand and on the concentrati on and the nature of the entering PP ligand. In the proposed mechanism. the rate determining step is the cleavage of one metal-olefin bond of the COD ligand. A rate-law is derived from the proposed mechanism. The evaluation o f the kinetic data gives the activation parameters which support an associa tive mechanism in the transition states. Both the observed rare constant an d the activation parameters show little variation with the chain length of the diphosphine ligand. (C) 1999 Elsevier Science S.A. All rights reserved.