Consecutive substitution in, and reduction of, η-allyl molybdenum(II) complexes, and a study of ligand exchange in a molybdenum(0) product

Abstract

Reactions of the allyl and 2-methylallyl complexes[Mo(CO)₂Cl(η-C₃H₄R)(NCMe)₂](R = H or Me) with ligands L = PMe₂Ph or PMePh₂ involve initial substitution to give [Mo(CO)₂Cl(η-C₃H₄R)L₂] followed by reduction to cis-[Mo(CO)₂L₄] or [Mo(CO)₂(NCMe)(PMePh₂)₃]. The reduction, which is first order in the concentrations of both molybdenum complex and L, is thought to involve initial nucleophilic attack on the allyl ligand. Two of the PMe₂Ph ligands in cis-[Mo(CO)₂(PMe₂Ph)₄], believed to be the mutually cis pair, undergo rapid dissociative exchange with free PMe₂Ph in solution at high temperatures: the exchange occurs without scramblig the cis pair of ligands with the trans pair. In the absence of free PMe₂Ph, slow decomposition at 353 K yields specifically the mer isomer of [Mo(CO)₃(PMe₂Ph)₃]. It is shown that this is the expected result if the labile Mo–P bonds are those to the cis pair of PMe₂Ph ligands.