Ferrocene is a sandwich complex consisting of two cyclopentadienyl rings bound to an iron atom. Other metallocenes are also known, but bis(cyclopentadienyl) complexes of scandium, i.e., Sc(II) scandocenes, had remained elusive so far. There have been reports of a heteroatomic scandium metallocene, but this complex was too reactive for crystallographic characterization.

Filipp Furche, William J. Evans, University of California, Irvine, USA, and colleagues have synthesized the first crystallographically characterizable bis(cyclopentadienyl) Sc(II) complex, (C₅H₂^tBu₃)₂Sc (pictured), which has tri-tert-butyl-substituted cyclopentadienyl ligands for steric protection. The team first prepared the iodide (C₅H₂^tBu₃)₂ScI as a precursor from KC₅H₂^tBu₃ and ScI₃.

This precursor was then reduced using KC₈ in hexane at room temperature under an Ar atmosphere, giving the desired (C₅H₂^tBu₃)₂Sc in a yield of about 75 %. The product was recrystallized and characterized using X-ray diffraction. The team found that the synthesized complex is, indeed, a scandocene and thermally stable. The cyclopentadienyl rings are not parallel as in ferrocene, but form a Cp–Sc–Cp angle of 170.6°. Of the six tert-butyl ligands, four are arranged in a staggered manner and two are eclipsed.

The researchers also reduced the precursor C₅H₃(SiMe₃)₂ScI using KC₈ in diethylether in the presence of 2.2.2-cryptand and obtained the complex salt [K(crypt)]{[C₅H₃(SiMe₃)₂]₂ScI}. X-ray crystallography showed that the anion in this compound is a bis(cyclopentadienyl) Sc(II) complex that retains an iodide ligand. However, this product is thermally unstable. Overall, the work extends the well-known bis(cyclopentadienyl) chemistry of other transition metals to scandium.

• Synthesis of Crystallographically Characterizable Bis(cyclopentadienyl) Sc(II) Complexes: (C₅H₂^tBu₃)₂Sc and {[C₅H₃(SiMe₃)₂]₂ScI}^1–,
  Joshua D. Queen, Lauren M. Anderson-Sanchez, Cary R. Stennett, Ahmadreza Rajabi, Joseph W. Ziller, Filipp Furche, William J. Evans,
  J. Am. Chem. Soc. 2024.
  https://doi.org/10.1021/jacs.3c11922