From order to disorder and return: Remarkable molecular and crystal dynamics in solid [(C5H5)2Co][PF6]

Crystalline [(C5H5)(2)Co][PF6] has been shown, by variable-temperature X-ray diffraction experiments (at 293, 243, and 313 K) carried out on the same crystal specimen, to undergo two fully reversible phase changes. The room-temperature ordered monoclinic crystal (form I) transforms, below 252 K, into another order monoclinic crystal (form II) with different relative orientation of the two independent [(C5H5)(2)Co](+) cations and into a semiplastic cubic system (form III) containing ordered PF6- anions and orientationally disordered [(C5H5)(2)Co](+) cations above 314 K.