Competing reaction mechanisms for the carbonylation of neutral palladium(II) complexes containing bidentate ligands: A theoretical study

The complete carbonylation mechanism for the model palladium(II) system, Pd(PH3)(CH3)(N-O) + CO --> Pd(PH3)(COCH3)(N-O) (N-O = NHCHCOO-), has been investigated employing nonlocal density functional theory (DFT) and second-order Moller-Plesset perturbation theory (MP2). Four possible mechanisms were identified, all involving the initial displacement of the phosphine ligand by carbon monoxide via a trigonal bipyramidal transition structure. Of these four mechanisms, the rate-determining methyl migration step was found to be lowest in energy when proceeding from a novel five-coordinate intermediate in which the palladium-nitrogen bond is weakened (2.385 Angstrom at the MP2 level of theory). This mechanism is consistent with the available experimental data. MP2 calculations using large basis sets predicted an overall exothermicity of -59.7 kJ/mol and a negligible activation energy of +6.0 kJ/mol with respect to the separated reactants. In addition, a novel transition structure that accounts for the isomerization of square-pyramidal d(8) complexes is presented.