Density Functional Study of the σ and π Bond Activation at the PdX (X = Sn, Si, C) Bonds of the (H₂PC₂H₄PH₂)PdXH₂ Complexes. Is the Bond Cleavage Homolytic or Heterolytic?

The mechanism for the activation of the σ bonds, the O−H of H₂O, C−H of CH₄, and the H−H of H₂, and the π bonds, the C⋮C of C₂H₂, CC of C₂H₄, and the CO of HCHO, at the PdX (X = Sn, Si, C) bonds of the model complexes (H₂PC₂H₄PH₂)PdXH₂ 5 has been theoretically investigated using a density functional method (B3LYP). The reaction is significantly affected by the electronic nature of the PdX bond, and the mechanism is changed depending on the atom X. The activation of the O−H bond with the lone pair electron is heterolytic at the PdX (X = Sn, Si) bonds, while it is homolytic at the PdC bond. The C−H and H−H bonds without the lone pair electron are also heterolytically activated at the PdX bonds independent of the atom X, where the hydrogen is extracted as a proton by the Pd atom in the case of X = Sn, Si and by the C atom in the case of XC because the nucleophile is switched between the Pd and X atoms depending on the atom X. In contrast, the π bond activation of C⋮C and CC at the PdSn bond proceeds homolytically, and is accompanied by the rotation of the (H₂PC₂H₄PH₂)Pd group around the Pd−Sn axis to successfully complete the reaction by both the electron donation from the π orbital to Sn p orbital and the back-donation from the Pd dπ orbital to the π* orbital. On the other hand, the activation of the CO π bond with the lone pair electron at the PdSn bond has two reaction pathways:  one is homolytic with the rotation of the (H₂PC₂H₄PH₂)Pd group and the other is heterolytic without the rotation. The role of the ligands controlling the activation mechanism, which is heterolytic or homolytic, is discussed.