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 CpNiC 12 H 8 – with MX 2 (M = Co, Ni) give the dinickelametallocene sandwich compounds (CpNiC 12 H 8 ) 2 M. We now report theoretical studies on the related bis(nickelacyclopentadienyl)metal derivatives (CpNiC 4 H 4 ) 2 M and bis(nickelaindenyl)metal derivatives (CpNiC 8 H 6 ) 2 M as well as the bis(nickelafluorenyl)metal derivatives. The structures of the lowest energy bis(nickelacyclopentadienyl) sandwich compounds (CpNiC 4 H 4 ) 2 M may be derived from those of the corresponding metallocenes Cp 2 M 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 η 5 -CpNiC 4 H 4 ligand. The spin states of the lowest energy (CpNiC 4 H 4 ) 2 M derivatives are similar to those of the corresponding metallocenes Cp 2 M, 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 (CpNiC 8 H 6 ) 2 M and then the experimentally known bis(nickelafluorene) sandwich compounds (CpNiC 12 H 8 ) 2 M lowers the energy of the higher spin state. As a result, the lowest energy (CpNiC 12 H 8 ) 2 Co structure is not the doublet spin state of Cp 2 Co and (CpNiC 4 H 4 ) 2 Co but instead a quartet spin state. This is in accord with experimental work showing (CpNiC 12 H 8 ) 2 Co to have a magnetic moment of ∼3.7 μ B, indicating three unpaired electrons and thus the predicted quartet spin state.