In recent years, ionic plastic crystals have attracted much attention. Many metallocenium salts exhibit plastic phases, but factors affecting their phase transitions are yet to be elucidated. To investigate these factors, we synthesized octamethylferrocenium salts with various counteranions [Fe(CMeH)]X ([1]X; X = B(CN), C(CN), N(CN), FSA (= (SOF)N), FeCl, GaCl and CPFSA (= CF(SOCF)N)) and elucidated their crystal structures and phase behavior. Correlations between the crystal structures and phase sequences, and the shapes and volumes of the anions are discussed. Except for [1][CPFSA], these salts exhibit phase transitions to plastic phases at or above room temperature (T = 298-386 K), and the plastic phases exhibit either NaCl- or anti-NiAs-type structures. X-ray crystal structure analyses of these salts at 100 K revealed that they have structures in which cations and anions are alternately arranged, with the exception of [1][CPFSA]. [1][CPFSA] exhibits a structure in which anions and cations are separately stacked to form columns. [1][N(CN)] exhibits a polar crystal structure that undergoes a monotropic phase transition to a centrosymmetric structure. The magnetic susceptibilities of room-temperature plastic crystals [1][GaCl] and [1][FeCl] were investigated; the latter exhibits a small ferromagnetic interaction at low temperatures.

We recently developed ionic liquids containing cationic sandwich complexes. However, salts of the sandwich complexes often exhibit ionic plastic phases with high melting points. To explore the boundaries between ionic liquids and plastic crystals of sandwich salts, we investigated, in detail, the phase behavior of ruthenium complexes [Ru(CH)(CHR)][X] ([C0][X]: R = H, [C1][X]: R = Me, [C2][X]: R = Et, [C4][X]: R = Bu). Among salts containing the anions PF, FSA, and B(CN), [C0][X] and [C1][X] are solids that exhibit plastic phases at or above room temperature, whereas [C2][X] and [C4][X] are mostly ionic liquids. Salts containing the C(CN) anion exhibited lower melting points than the other salts. X-ray crystallography reveals that the cations and anions in most of these salts are arranged alternately in the solid state. However, in the case of [C0][C(CN)], the cations and anions are stacked independently, thereby providing weaker cation-anion interactions that account for the relatively low melting point of this salt.