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.