Stability and possible multiple metal-metal bonding in tetranuclear sandwich complexes of cyclooctatetraene ligand

Geometry optimizations have been performed on the M-4(COT)(2) (M = Cr-Ni, Pd and COT = C8H8) complexes by means of DFT method using BP86-D functional combined to the TZP basis set. The M-4 moiety encapsulated between two COT ligands tends to establish M-L and M-M bonding in relationship with the metal nature and the spin state. In accordance with the coordination modes, the COT behaves as neutral or dianionic ligand. The calculations showed that the various complexes are found to have a low, intermediate, or high-spin ground state. The multiple metal-metal bonding is identified by exploring the bond distances, MO plots, and Wiberg bond index, which vary according to the valence electron count of the M-4 core.The Ziegler-Rauk energy decomposition analysis scheme was employed to characterize the geometry distortion and steric interaction (electrostatic and Pauli) and orbital interaction terms in the total bonding energy. The results showed that the interaction terms in all studied complexes are governed by one third covalent and two third ionic characters, in agreement with the Delta E-elstat (electrostatic) and Delta E-orb (orbital) contributions, respectively, into the total attractive interaction (Delta E-elstat + Delta E-orb).