Ligand substitution kinetics in M(CO)_4 (#eta#~(2:2)-1,5-cyclooctadiene) complexes (M velence Cr, Mo, W)—substitution of 1,5-cyclooctadiene by bis(diphenylphosphihino)alkanes

摘要

The thermal substitution kinetics of 1,5-cryclooctadiene (COD) by bis(diphenylphosphino)alkanes (PP), (C_6H_5)_2P(CH_2)_nP(C_6H_5)_2 (n = 1,2,3) in M(CO)_4(#eta#~(2:2)-COD) complexes (M velence Cr, Mo, W), were studied by quantitative FT-IR spectroscopy. The reaction rate exhibits first-order dependence on the concentration of the starting complex, and the observed rate constant depends on the concentraton of the leaving COD ligand and on the concentration 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 of the kinetic data gives the activation parameters which support an associative mechanism in the transition states. Both the observed rate constant and the activation parameters show little variation with the chain length of the diphosphine ligand.