Exposure of the solid zwitterion [Co^III(η⁵-C₅H₄CO₂ H)(η⁵-C₅H₄CO₂)] to hydrated vapours of volatile acids (HCl, CF₃CO₂H, HBF₄) or bases (NH₃, NMe₃, NH₂Me) quantitatively produces the corresponding salts; the heterogeneous reactions are fully reversible, as the acid or base molecules can be removed by thermal treatment, regenerating the starting material.