Dinickelametallocenes: Sandwich Compounds of the First-Row Transition Metals (M = Fe, Co, Ni) with Two Pentahapto Planar Nickelacycle Ligands

Buchalski and co-workers showed in 2008 that reactions of the nickelafluorenyl anion CpNiC₁₂H₈^– with MX₂ (M = Co, Ni) give the dinickelametallocene sandwich compounds (CpNiC₁₂H₈)₂M. We now report theoretical studies on the related bis­(nickelacyclopentadienyl)­metal derivatives (CpNiC₄H₄)₂M and bis­(nickelaindenyl)­metal derivatives (CpNiC₈H₆)₂M as well as the bis­(nickelafluorenyl)metal derivatives. The structures of the lowest energy bis­(nickelacyclopentadienyl) sandwich compounds (CpNiC₄H₄)₂M may be derived from those of the corresponding metallocenes Cp₂M by replacing a CH group in each Cp ring with an isolobal CpNi unit. The Ni–M distances of ∼2.5 Å indicate formal single bonds and thus a pentahapto η⁵-CpNiC₄H₄ ligand. The spin states of the lowest energy (CpNiC₄H₄)₂M derivatives are similar to those of the corresponding metallocenes Cp₂M, namely, singlet, doublet, and triplet for M = Fe, Co, and Ni, respectively. Fusion of benzene rings to the nickelacyclopentadienyl rings to give first the bis­(nickelaindene) sandwich compounds (CpNiC₈H₆)₂M and then the experimentally known bis­(nickelafluorene) sandwich compounds (CpNiC₁₂H₈)₂M lowers the energy of the higher spin state. As a result, the lowest energy (CpNiC₁₂H₈)₂Co structure is not the doublet spin state of Cp₂Co and (CpNiC₄H₄)₂Co but instead a quartet spin state. This is in accord with experimental work showing (CpNiC₁₂H₈)₂Co to have a magnetic moment of ∼3.7 μ_B, indicating three unpaired electrons and thus the predicted quartet spin state