First-Row Transition Metals in Binuclear Cyclopentadienylmetal Derivatives of Tetramethyleneethane: η³,η³ versus η⁴,η⁴ Ligand–Metal Bonding Related to Spin State and Metal–Metal Bonds

Abstract

The tetramethyleneethane (TME) ligand is found in the η³,η³ complex <i>tran</i>s<i>-</i>(η³,η³-TME)­Ni₂Cp₂ and the η⁴,η⁴ complex <i>tran</i>s<i>-</i>(η⁴,η⁴-TME)­Co₂Cp₂, which have both been synthesized and structurally characterized by X-ray crystallography. The structures of the complete series of (TME)­M₂Cp₂ derivatives of the first-row transition metals from Ti to Ni have now been investigated by density functional theory. The experimentally known nickel and cobalt complexes are found to have closed-shell electronic ground states. The lowest energy structure for the iron complex is the triplet spin state <i>trans</i>-(η⁴,η⁴-TME)­Fe₂Cp₂ structure with geometry similar to that of the lowest energy cobalt structure. All of the low-energy (TME)­M₂Cp₂ structures for manganese and the first-row transition metals to the left of manganese (Cr, V, and Ti) exhibit <i>cis</i>-(η⁴,η⁴-TME)­M₂Cp₂ stereochemistry, thereby providing the possibility for direct metal–metal interactions. Vanadium is the only first-row transition metal to the left of cobalt where the lowest energy (TME)­M₂Cp₂ structure is a singlet spin state, suggesting limited applicability of the 18-electron rule in these systems. The frontier molecular orbitals of these <i>cis</i>-(η⁴,η⁴-TME)­M₂Cp₂ systems have been examined in order to provide insight regarding metal–metal bonding. Thus, low-energy <i>cis-</i>(η⁴,η⁴-TME)­Mn₂Cp₂ structures are found in both quintet and triplet spin states with formal MnMn double bonds. The lowest energy <i>cis</i>-(η⁴,η⁴-TME)­Cr₂Cp₂ and <i>cis</i>-(η⁴,η⁴-TME)­Ti₂Cp₂ structures have triplet spin states with a formal CrCr triple bond and a formal TiTi double bond, respectively. The lowest energy <i>cis</i>-(η⁴,η⁴-TME)­V₂Cp₂ structure is a singlet structure with a formal VV triple bond