Oxidative addition of a C–H σ bond to M(PH3)2 (M = Pd or Pt). An ab initio molecular orbital study on the chelate phosphine effect

All the reactants, transition states and products of the oxidative addition of a C–H σ bond to M(PH 3 ) 2 (M = Pd or Pt) were optimized at the second-order Møller-Plesset perturbation (MP2) level and the energy changes in the reaction were evaluated by ab initio molecular orbital/MP4, SD-CI (single- and double-excitation configuration interaction) and CCD (coupled cluster with double substitutions) methods. In the model complexes of a monodentate phosphine this reaction requires a considerably high activation energy (E a ), 30 and 37 kcal mol -1 for M = Pt and Pd (MP4SDQ values), respectively. However, this reaction proceeds with a much lower activation energy when two PH 3 ligands are positioned so as to mimic a chelate phosphine, e.g. a diphosphinomethane; for instance, E a = 20.0 and 3.8 kcal mol -1 for M = Pd and Pt, respectively. This significant effect of a chelate phosphine is discussed in detail from the point of view of the frontier orbital, electronic reorganization and distortion energies of M(PH 3 ) 2 .