The steric nature of the bite angle.

According to this electronic model, the transition state (TS) is stabilized by donor–acceptor orbital interactions from metal d orbitals to the substrate s*C X, which become more stabilizing as the metal–ligand d hybrid orbital is pushed up at smaller bite angles. Here, we provide evidence that the mechanism through which the bite angle affects barrier heights (and also the twist angle in the TS, see Scheme 1) is steric in nature, rather than electronic. This insight is of direct relevance for developing correct, rational design principles for catalysts. Our evidence is based on relativistic density functional theory (DFT) analyses with the ADF program at ZORABLYP/TZ2P (see reference [6] for benchmark studies) of a series of oxidative insertion (OxIn) reactions of model catalysts Pd, Pd ACHTUNGTRENNUNG(PH3)2 and PdACHTUNGTRENNUNG[PH2 ACHTUNGTRENNUNG(CH2)nPH2] (denoted as PdACHTUNGTRENNUNG[PnP]) with n=1–6 and model substrates H3C X with X= H, CH3, and Cl. To obtain a qualitative, physical understanding of how trends in catalytic activity (i.e., OxIn barrier height) are determined by the ligand and catalyst structure, we have carried out an analysis of the reaction profiles using the activation-strain model of chemical reactivity. In this model, the potential-energy surface DE(z) is decomposed, along the reaction coordinate z (obtained from intrinsic reaction coordinate (IRC) calculations), into the strain associated with deforming the substrate, DEstrainACHTUNGTRENNUNG[substr](z), and the catalyst, DEstrainACHTUNGTRENNUNG[cat](z), plus the actual interaction DEint(z) between these two deformed reactants [Eq. (2)]: