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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).