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

The tetramethyleneethane (TME) ligand is found in the η3,η3 complex trans-(η3,η3-TME)­Ni2Cp2 and the η4,η4 complex trans-(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 trans-(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 cis-(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 cis-(η4,η4-TME)­M2Cp2 systems have been examined in order to provide insight regarding metal–metal bonding. Thus, low-energy cis-(η4,η4-TME)­Mn2Cp2 structures are found in both quintet and triplet spin states with formal MnMn double bonds. The lowest energy cis-(η4,η4-TME)­Cr2Cp2 and cis-(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 cis-(η4,η4-TME)­V2Cp2 structure is a singlet structure with a formal VV triple bond.