M(C6X6Li6)2 (M = Cr, Mo, W; X = O, S): Transition-metal sandwich complexes with π-aromatic C6X6Li6 ligands

The geometrical and electronic structures of a new class of transition-metal sandwich complexes M(C6X6Li6)2 (M = Cr, Mo, W; X = O, S) have been systematically investigated in this work at DFT B3LYP and PBE1PBE levels. Both the D2d M(C6O6Li6)2 and D6d M(C6S6Li6)2 (M = Cr, Mo, W) complex series are found to conform to the 18-electron rule similar to dibenzenechromium Cr(C6H6)2 and are stable in thermodynamics. Molecular orbital analyses indicate that effective d–π coordination bonds are formed between the partially filled nd atomic orbitals of transition-metal centers (Cr, Mo, W) and the π molecular orbitals of the C6X6Li6 ligands (X = O, S). Electrostatic interactions between the two interlaced C6X6Li6 (X = O and S) ligands also contribute to the high stability of these sandwich complexes. The calculated infrared spectra of these complexes are presented to facilitate their future experimental characterizations.