Topotactic reversible phase transition between room-temperature and a new low-temperature modification in the MCl3(THF)(3) system

Both single crystals of VCl3(THF)(3) as well as isotypic cocrystals of the composition MCl3(THF)(3), M = Ti/V approximate to 1/3, undergo a topotactic reversible phase transition to a hitherto unknown low-temperature modification. The close relationship between this new structure and the room-temperature phase determined by Cotton et al. is discussed from the molecular and the intermolecular point of view. Both modifications are built up by conformationally very similar molecules which change their arrangement during the phase transition. Lattice energy calculations confirm that these two alternative arrangements correspond to minima of almost the same packing energy.