DISPLACEMENT KINETICS OF 1,5-CYCLOOCTADIENE FROM M(CO)(4)(ETA(2 2)-1.5-CYCLOOCTADIENE) (M=CR, MO, W) BY BIS(DIPHENYLPHOSPHINO)ALKANE/

The thermal substitution kinetics of 1,5-cyclooctadiene (COD) from M(C O)(4)(eta(2:2)-COD) (M = Cr, Mo, W) by bis(diphenylphosphino)alkane, ( C6H5)(2)P(CH2)(n)-P(C6H5)(2) (n = 1, 2, 3), was studied by quantitativ e FT-IR spectroscopy. The reaction rate exhibits first-order dependenc e on the concentration of starting complex and the observed rate const ant depends on the concentrations of both the leaving COD and entering diphosphine ligand. In the proposed mechanism the rate determining st ep is the cleavage of one of two metal-olefin bonds. A rate-law is der ived from the proposed mechanism. The evaluation of the kinetic data g ives the activation parameters which tend to suggest an associative me chanism for the molybdenum and tungsten complexes, and a dissociative mechanism for the chromium complexes in transition states. The observe d rate constant is found to be dependent on the concentration and the nature of the entering diphosphine ligand.