First-Row Transition Metals in Binuclear Cyclopentadienylmetal Derivatives of Tetramethyleneethane: η~3,η~3 versus η~4,η~4 Ligand? Metal Bonding Related to Spin State and Metal?Metal Bonds

摘要

The tetramethyleneethane (TME) ligand is found in the η~3,η~3 complex trans-(η~3,η~3-TME)Ni_2Cp_2 and the η~4,η~4 complex trans-(η~4,η~4-TME)- Co_2Cp_2, which have both been synthesized and structurally characterized by X-ray crystallography. The structures of the complete series of (TME)M_2Cp_2 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-(η~4,η~4-TME)Fe_2Cp_2 structure with geometry similar to that of the lowest energy cobalt structure. All of the low-energy (TME)M_2Cp_2 structures for manganese and the first-row transition metals to the left of manganese (Cr, V, and Ti) exhibit cis-(η~4,η~4- TME)M_2Cp_2 stereochemistry, thereby providing the possibility for direct metal?metal interactions. Vanadium is the only firstrow transition metal to the left of cobalt where the lowest energy (TME)M_2Cp_2 structure is a singlet spin state, suggesting limited applicability of the 18-electron rule in these systems. The frontier molecular orbitals of these cis-(η~4,η~4-TME)M_2Cp_2 systems have been examined in order to provide insight regarding metal?metal bonding. Thus, low-energy cis-(η~4,η~4- TME)Mn_2Cp_2 structures are found in both quintet and triplet spin states with formal Mn＝Mn double bonds. The lowest energy cis-(η~4,η~4-TME)Cr_2Cp_2 and cis-(η~4,η~4-TME)Ti_2Cp_2 structures have triplet spin states with a formal Cr≡Cr triple bond and a formal Ti＝Ti double bond, respectively. The lowest energy cis-(η~4,η~4-TME)V_2Cp_2 structure is a singlet structure with a formal V≡V triple bond.