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(C9H7)2] (1) and [Be{1,3‐(SiMe3)2C9H5}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. The final puzzle piece fits in! Beryllium is the last of the non‐radioactive s‐block metals to be coordinated to indenyl ligands. Milling K[C9H7] with BeBr2 produces the indenyl complex [Be(C9H7)2]. Its crystal structure reveals mixed η5/η1 ring coordination. The monomeric “half‐sandwich” complex [Be{1,3‐(SiMe3)2C9H5}Br] is available via the same method.