Density Functional Study of the π Bond Activation at the PdSn Bond of the (H₂PC₂H₄PH₂)PdSnH₂ Complex. Why Do the (H₂PC₂H₄PH₂)Pd and SnH₂ Counterparts Mutually Rotate?

The activation mechanism of the π bonds, the nonpolar C⋮C of C₂H₂ and the polar CO of HCHO and the C⋮N of HCN, at the PdSn bond of the model complex (H₂PC₂H₄PH₂)PdSnH₂ is theoretically examined using a density functional method (B3LYP). For the nonpolar ethyne C⋮C π bond, the reaction proceeds by the homolytic mechanism supported by the rotation of the (H₂PC₂H₄PH₂)Pd group around the Pd−Sn axis. The charge transfers, the electron donation from the ethyne π orbital to the Sn p orbital, and the back-donation from Pd dπ orbital to the ethyne π* orbital successfully occur step by step to complete the π bond breaking during the reaction. Without the rotation of the (H₂PC₂H₄PH₂)Pd group, the first charge transfer from the ethyne π orbital to the Sn p orbital is too weak to break the π bond. On the other hand, for the strongly polarized formaldehyde CO π bond, the donation of lone pair electron on the CO oxygen to the Sn p orbital is so strong that both the nucleophilicity of the Pd atom and the electrophilicity of CO carbon are significantly enhanced through the Pd(dπ)−Sn(pπ) orbital, and the CO π bond is broken by the heterolytic mechanism with the electrophilic attack of the CO carbon to the Pd atom. In this mechanism, the rotation of the (H₂PC₂H₄PH₂)Pd group is not necessary. However, when the CO π bond approaches the Sn atom in a η²-fashion, the reaction proceeds by the homolytic mechanism with the rotation of the (H₂PC₂H₄PH₂)Pd group, which is similar to the case of ethyne. In the case of hydrogen cyanide, where the C⋮N π bond has to approach the Sn atom in a η²-fashion to break its π bond, only the homolytic pathway with the rotation of the (H₂PC₂H₄PH₂)Pd group exists. The first charge transfer from the substrate to the Sn p orbital plays a key role in determining the mechanism, and it is strengthened enough to break the π bond by the rotation of the (H₂PC₂H₄PH₂)Pd group in the homolytic mechanism. The potential energy surface of the activation reaction was quite smooth with a small energy barrier even in the homolytic mechanism with the rotation of the (H₂PC₂H₄PH₂)Pd group due to the successive stepwise process. The ligand effects on the activation activity are also discussed.