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

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.