Mechanochemical production of an allyl potassium / calcium complex that polymerizes isoprene at room temperature. 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)2C3 H3]– ) with CaI2 yields 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.