The mechanism of displacement of diene from [M(η4-diene)(CO)4](M = Cr, Mo, or W, diene = norbornadiene; M = Mo, diene = cyclo-octa-1,5-diene) and the cis⇌trans isomerization of [M(CO)4L2] complexes [L = PBu3, P(OMe)3, or P(OPh)3]

Abstract

Kinetic studies are reported of the displacement of the diene from [M(η⁴-diene)(CO)₄][diene = norbornadiene, M = Cr, Mo, or W; diene = cyclo-octa-1,5-diene (cod), M = Mo] by phosphorus ligands [L = PBu₃, P(OMe)₃, or P(OPh)₃] to give complexes [M(CO)₄L₂]. With the exception of [Mo(cod)(CO)₄], all reactions exhibit a bimolecular rate law of the type –d[A]/dt=K_obs.[A][L], where A =[M(diene)(CO)₄] which is interpreted mechanistically as a stepwise ligand-assisted dechelation of the diene; k_obs. may be seen to increase in the order Cr < W Mo, while the ordering in terms of ligand [P(OPh)₃ < P(OMe)₃ < PBu₃] reflects the increasing order of ligand nucleophilicity. cis–trans Isomerization of [M(CO)₄L₂](L = PBu₃, P(OMe)₃, or P(OPh)₃; M = Cr, Mo, or W) has also been studied kinetically. With the exception of [Mo(CO)₄{P(OPh)₃}₂], all isomerizations are intramolecular. Rates of isomerization increase in the order Mo < W < Cr and in the order P(OPh)₃ < P(OMe)₃ < PBu₃, while K_eq(=[trans]/[cis]) increases in the order W ≈ Mo Cr and in the order P(OPh)₃ < P(OMe)₃ < PBu₃. These results are interpreted in terms of both steric and electronic factors.