Bonding analysis of the donor-acceptor sandwiches CpE-MCp (E = B, Al, Ga; M = Li, Na, K; Cp = η⁵-C₅H₅).

The nature of E-M bonds in CpE-MCp (E = B, Al, Ga; M = Li, Na, K; Cp = η (5)-C5H5) donor-acceptor sandwiches was studied using the atoms in molecules (AIM) theory, electron localization function (ELF), energy decomposition analysis (EDA), and natural bond orbital analysis (NBO) methods. Both topological and orbital analysis show that the E atom determines the bond strength of the E-M bonds, while the M atom has little influence on it. E-M bond strength decreases in the order E = B, Al, and Ga. The EDA analysis shows that the electrostatic character decreases following the sequence E = B > Al > Ga. Not only the s orbital, but also the p orbital of the E/M atom participates in formation of the E-M bond. The interactions of E and M with Cp are different. The M-Cp interaction is purely electrostatic while the E-Cp interaction has a partly covalent character.