Intercalation of Transition Metals into Stacked Benzene Rings: A Model Study of the Intercalation of Transition Metals into Bilayered Graphene

Structures of neutral metal–dibenzene complexes, M(C₆H₆)₂ (M = Sc–Zn), are investigated by using Møller–Plesset second order perturbation theory (MP2). The benzene molecules change their conformation and shape upon complexation with the transition metals. We find two types of structures: (i) stacked forms for early transition metal complexes and (ii) distorted forms for late transition metal ones. The benzene molecules and the metal atom are bound together by δ bonds which originate from the interaction of π-MOs and d orbitals. The binding energy shows a maximum for Cr(C₆H₆)₂, which obeys the 18-electron rule. It is noticeable that Mn(C₆H₆)₂, a 19-electron complex, manages to have a stacked structure with an excess electron delocalized. For other late transition metal complexes having more than 19 electrons, the benzene molecules are bent or stray away from each other to reduce the electron density around a metal atom. For the early transition metals, the M(C₆H₆) complexes are found to be more weakly bound than M(C₆H₆)₂. This is because the M(C₆H₆) complexes do not have enough electrons to satisfy the 18-electron rule, and so the M(C₆H₆)₂ complexes generally tend to have tighter binding with a shorter benzene–metal length than the M(C₆H₆) complexes, which is quite unusual. The present results could provide a possible explanation of why on the Ni surface graphene tends to grow in a few layers, while on the Cu surface the weak interaction between the copper surface and graphene allows for the formation of a single layer of graphene, in agreement with chemical vapor deposition experiments