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

Abstract

Density functional theory has been used to examine the dimetallocene‐like dicycloheptatrienyl dimetal compounds of the second‐row transition metals (C7H7)2M2 (M = Ru, Tc, Mo, Nb, Zr). The lowest energy (C7H7)2Mo2 structure is a coaxial structure with terminal η7C7H7 rings, whereas the lowest energy (C7H7)2M2 structures (M = Ru, Tc, Nb, Zr) are perpendicular structures with bridging η4,η4C7H7 rings except for the perpendicular (η4,η3C7H7)2Ru2 structure. The metal–metal bond orders in the (C7H7)2M2 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 (η7C7H7)2M2 structures the formal bond orders are two for M = Tc and three for M = Mo. For the perpendicular structures both (η4,η3C7H7)2Ru2 and (η4,η4C7H7)2Tc2 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)2Zr2 system the perpendicular structure has a formal ZrZr double bond and the coaxial structure has a very long (∼3.5 Å) ZrZr bond indicating only 12‐ to 14‐electron configurations for the zirconium atoms.