Covalent or not? Energy decomposition analysis of metal-metal bonding in alkaline-Earth dimetallocene complexes

The geometries, metal-metal bond dissociation energies of Group 2 dimetallocene complexes M2(η5-C5H5)2 (M = Be, Mg, Ca, Sr and Ba) have been calculated using density functional theory at the BP86 level with TZ2P basis sets. The nature of metal-metal bonding has been analyzed with an energy decomposition method. The results revealed that the M-M binding interactions in these nonpolar bonding alkaline-Earth metal complexes have more ionic character than covalent character, rather than the sole covalent bond. Molecular orbital calculations indicate that, for the heavier alkaline-Earth metal (Ca, Sr and Ba) compounds, substantial (n − 1)d character is found in the δ bonding between metal and cyclopentadiene (Cp) ring, while there is few δ bonding interaction between M-M bond due to longer M-M bond distance.