Phosphine substitution in (.eta.5-cyclopentadienyl)bis(triphenylphosphine)cobalt(I): evidence for a dissociative mechanism

Abstract

The substitution of trimethylphosphine for triphenylphosphine in (η⁵-cyclopentadienyl)bis(triphenylphosphine)cobalt(I) (1) to form (η⁵-cyclopentadienyl)(trimethylphosphine)(triphenylphosphine)cobalt(I) was studied at -60°C in an NMR spectrometer. Kinetic measurements show the process to be first order in 1 and zero order in PMe₃; added PPh₃ strongly inhibits the reaction rate. This information indicates the reaction proceeds by rapid reversible phosphine dissociation through the unsaturated CpCo(PPh₃) intermediate. The rate for generation of that intermediate, k₁, is 1.15 x 10^-3s^-1 while the ratio of rate constant k₂ (for conversion of intermediate to products) to k_-1 (return to starting materials) is 4 at -60°C. Possible structures for CpCo(L) are also discussed in light of recent indications that the linear structure has a triplet ground state.