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 μB, indicating three unpaired electrons and thus the predicted quartet spin state.