Isomerization and Chelation Studies of Dangling Phosphine Complexes of Group 6

Vinyl addition reactions, catalyzed by 2,2\u27-azobis(isobutyronitrile) (AIBN), have been used to prepare six pentacarbonyl complexes of group 6, each of which contains a dangling Ph2PCH2CH2P(p-tol)2 ligand. Each of the six complexes undergoes slow isomerizationand chelation at 55 °C in perdeuterated toluene: The reactions were monitored by 31P{1H} NMR spectroscopy. The isomerization reactions are reversible in solution, while the chelated products form irreversibly with the loss of carbon monoxide. The rate constants, K1 and K-1, have been determined for the three pairs of linkage isomers. Equilibrium constants for the chromium and tungsten isomerization reactions were determined from the rate constants. The molybdenum isomerization reactions were sufficiently fast to allow the equilibrium constant to be determined directly. In addition, rate constants K2 and K3, were determined for the chromium and molybdenum chelation reactions. The rate of chelation for the tungsten complexes was too slow to determine. Rate constants and equilibrium constants are shown below: Rate constants for the isomerization and chelation of the chromium complexes are nearly the same, consistent with dissociative (D) mechanisms for each reaction. Rate constants for isomerization of the molybdenum and tungsten complexes are much larger than those for chelation, suggestive of an associative component in the former reaction. Even so, bond-breaking would appear to be more important than bond-making. Thus, the isomerization is best described as proceeding through an interchange dissociative mechanism (ID)