Kinetics and mechanism of reactions of cis-(piperidine)(L)Mo(CO)4 complexes with phosphines and phosphites

The kinetics and mechanism of ligand-exchange reactions of cis-(pip)(L)Mo(CO)(4) complexes (pip = piperidine; L = P(OCH3)(3), P(OC2H5)(3), P(OCH(CH3)(2))(3), P(OC6H5)(3), P(C6H5)(3), P(OCH2)(3)CCH3, P(C6H12N3); L' = P(OCH(CH3)(2))(3), and L = P(OCH(CH3)(2))(3); L' = P(OCH3)(3), P(OC2H5)(3), P(OC6H5)(3), P(C6H5)(3), P(OCH2)(3)CCH3, P(n-C4H9)(3), cis-(pip)(L)Mo(CO)(4) + L' --> cis,trans-(L)(L')Mo(CO)(4) + pip (2) have been studied. Both the reactants and products (15 complexes) for these reactions have been synthesized and characterized. All reactions cleanly follow a mechanism involving initial Mo-pip hond-breaking, followed by competition between pip and L' for the resulting five-coordinate [(L)Mo(CO)(4)] intermediate. These data complete the study of these systems for the series of metals, Cr, Mo, W. The activation parameters for Mo-pip bond-breaking in the reactions of cis-(pip)(L)Mo(CO)(4) with L' have been obtained. Where L is a trialkyl phosphite, both the enthalpies and entropies of activation are in very close agreement. These values, when compared to those for complexes where L is a trialkyl phosphine, support intramolecular N-H ... O-P-hydrogen bonding in the phosphite complexes, but not in the phosphines. The data are interpreted in terms of a "fundamental" ligand exchange, involving [(L)Mo(CO)(4)] and L'. The steric outcome of these reactions also is influenced by intramolecular hydrogen bonding. The reliability of the reported activation parameters in these and related systems is discussed. The consistency from complex to complex of the reported activation parameters strongly indicates that they can be accepted with confidence.