Understanding the chemical bonding in sandwich complexes of transition metals coordinated to nine-membered rings: energy decomposition analysis and the donor–acceptor charge transfers

DFT calculations on sandwich complexes of the type (C9H9)2M and (C6N3H6)2M (M = Sc–Ni) are carried out in gas phase and tetrahydrofurane solvent (THF) using BP86 and B3LYP functionals. The nine-membered (C9H9)− and (C6N3H6)− ligand anions behave differently regarding their coordination to the M2+ metal cation. The computed structural and energetic parameters are weakly influenced by the inclusion of THF solvent. We report that the metal–ligand separation are sensitive to the electron-donation and electron π-backdonation as highlighted by the populations of the occupied and empty orbitals of the [(C9H9)−]2 and [(C6N3H6)−]2 anions and the electronic configuration of the M2+ cation. The results showed that the [(C9H9)−]2 and [(C6N3H6)−]2 ligands behave differently in terms of bonding, coordination mode and donation and π-backdonation properties in relationship with the metal cation radius, the number of the atoms involved in the coordination and their nature (either carbon or nitrogen atoms). The interactions are dominated by electrostatic and orbital terms each contributing approximately half into the total attractive energy. The donor–acceptor between NBOs identified within the NEDA scheme show the importance of σ → LP charge transfers contrary to those related to LP → σ* ones in line with their occupations and second perturbative energy E2.