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Abstract 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⇄ η³shifts), have remained unknown. Standard metathetical approaches to their synthesis (e.g., with K[Ind′] + BeX₂in 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 BeBr₂and 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⁻¹higher in energy. 

Abstract Without solvents present, the often far‐from‐equilibrium environment in a mechanochemically driven synthesis can generate high‐energy, non‐stoichiometric products not observed from the same ratio of reagents used in solution. Ball milling 2 equiv. K[A’] (A’=[1,3‐(SiMe₃)₂C₃H₃]⁻) with CaI₂yields a non‐stoichiometric calciate, K[CaA’₃], which initially forms a structure (1) likely containing a mixture of pi‐ and sigma‐bound allyl ligands. Dissolved in arenes, the compound rearranges over the course of several days to a structure (2) with only η³‐bound allyl ligands, and that can be crystallized as a coordination polymer. If dissolved in alkanes, however, the rearrangement of1to2occurs within minutes. The structures of1and2have been modeled with DFT calculations, and2initiates the anionic polymerization of methyl methacrylate and isoprene; for the latter, under the mildest conditions yet reported for a heavy Group 2 species (one‐atm pressure and room temperature).