Metal-metal bonding in biscycloheptatrienyl dimetal compounds of the second-row transition metals

摘要

Density functional theory has been used to examine the dimetallocene-like dicycloheptatrienyl dimetal compounds of the second-row transition metals (C7H7)(2)M-2 (M=Ru, Tc, Mo, Nb, Zr). The lowest energy (C7H7)(2)Mo-2 structure is a coaxial structure with terminal (C7H7)-C-7 rings, whereas the lowest energy (C7H7)(2)M-2 structures (M=Ru, Tc, Nb, Zr) are perpendicular structures with bridging (4),(C7H7)-C-4 rings except for the perpendicular ((4),(C7H7)-C-3)(2)Ru-2 structure. The metal-metal bond orders in the (C7H7)(2)M-2 structures (M=Ru, Tc, Mo, Nb), as determined by analysis of their frontier molecular orbitals, suggest preferred 16- rather than 18-electron configurations for the central metal atoms. Thus, in the coaxial ((C7H7)-C-7)(2)M-2 structures the formal bond orders are two for M=Tc and three for M=Mo. For the perpendicular structures both ((4),(C7H7)-C-3)(2)Ru-2 and ((4),(C7H7)-C-4)(2)Tc-2 have 16-electron configurations with metal-metal single bonds owing to the different modes of bonding of the bridging C7H7 rings in the two structures. For the (C7H7)(2)Zr-2 system the perpendicular structure has a formal ZrZr double bond and the coaxial structure has a very long (approximate to 3.5 angstrom) ZrZr bond indicating only 12- to 14-electron configurations for the zirconium atoms.