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.