Di(indenyl)beryllium

The bonding in beryllocene, [BeCp₂ ], took decades to establish, owing to its unexpected mixed hapticity structure (i.e., [Be(η⁵ -Cp)(η¹ -Cp)]). Beryllium complexes containing the indenyl ligand, which is a close relative of the cyclopentadienyl anion, but which is also known to exhibit its own bonding peculiarities (e.g., facile η⁵ ⇄ η³ shifts), have remained unknown. Standard metathetical approaches to their synthesis (e.g., with K[Ind'] + BeX₂ in an ether solvent) give rise to intractable oils from which nothing identifiable can be isolated. In contrast, mechanochemical preparation, involving the solvent-free grinding of BeBr₂ and potassium indenides, leads to the production of discrete (indenyl)beryllium complexes, including [Be(C₉ H₇ )₂ ] (1) and [Be{1,3-(SiMe₃ )₂ C₉ H₅ }Br] (2). The former displays η⁵ /η¹ -coordinated ligands in the solid state, but DFT calculations indicate that an η⁵ /η⁵ -conformation is less than 5 kcal mol^-1 higher in energy.