Aromatic C–H σ‑Bond Activation by Ni⁰, Pd⁰, and Pt⁰ Alkene Complexes: Concerted Oxidative Addition to Metal vs Ligand-to-Ligand H Transfer Mechanism

C–H σ-bond activation of arene (represented here by benzene) by the Ni⁰ propene complex Ni⁰(IMes)­(C₃H₆) (IMes = 1,3-dimesitylimidazol-2-ylidene), which is an important elementary step in Ni-catalyzed hydroarylation of unactivated alkene with arene, was investigated by DFT calculations. In the Ni⁰ complex, the C–H activation occurs through a ligand-to-ligand H transfer mechanism to yield Ni^II(IMes)­(C₃H₇)­(Ph) (C₃H₇ = propyl; Ph = phenyl). In Pd⁰ and Pt⁰ analogues, the activation occurs through concerted oxidative addition of the C–H bond to the metal. Analysis of the electron redistribution during the C–H activation highlights the difference between the two mechanisms. In the ligand-to-ligand H transfer, charge transfer (CT) occurs from the metal to the benzene. However, the atomic population of the transferring H remains almost constant, suggesting that different CT simultaneously occurs from the transferring H to the LUMO of propene. The electron redistribution contrasts significantly with that found for Pd⁰ and Pt⁰, in which CT occurs only from the metal to the benzene. Preference for ligand-to-ligand H transfer over concerted oxidative addition in the Ni⁰ complex is shown to be due to the smaller atomic radius of Ni in comparison to those of Pd and Pt and the smaller Ni^II–H bond energy relative to the Pd^II–H and Pt^II–H energies. Interestingly, the bulky ligand accelerates the ligand-to-ligand H transfer in the Ni⁰ complex by decreasing the distance between the coordinated benzene and alkene substrates. Thus, the Gibbs activation energy (ΔG°^⧧) decreases in the case of cyclic-alkylaminocarbene with bulky substituents (CACC-K3), while the ΔG°^⧧ values are similar in X-Phos, IMes, and nonsubstituted cyclic alkylaminocarbene (CAAC-K0). An electron-withdrawing substituent on the arene accelerates the C–H activation by favoring the metal to arene CT.