Activation of C ( sp 3 ) – X , C ( sp 2 ) – X , and C ( sp ) –

We systematically investigate the activation of a series of variously hybridized bonds in simple model organic compounds by palladium. These comprise the C(sp3)–X, C(sp2)–X, and C(sp)–X bonds in X–substituted ethane, ethylene, and acetylene, respectively, with X = H, CH3, and Cl. Although the bonds become stronger from sp3 to sp hybridization, the activation barriers become lower. Our analysis show that the main reason for this is the increase in back-donation into the lower lying empty &*C–X LUMO orbital, and to a lesser extent, interactions with the "C–C and "*C–C orbitals. A second effect is a decrease in steric repulsion as the substrate becomes smaller in going towards sp-hybridization. We include model ligand effects of the PdCl–, Pd(PH3)2, and Pd(PH3)2Cl– catalytic compounds. We mainly focus on the effect the ligands have along the reaction path compared to the bare palladium, and in general see that the effect of the ligands on the interaction between catalyst and substrate becomes progressively stronger along the series of increased hybridization.