Reversible dimerization of C₆₀ molecules in the crystal structure of the bis(arene)chromium fulleride [Cr(C₇H₈)]₂C₆₀

Bis(toluene)chromium fulleride, [Cr(C(7)H(8))(2)]C(60), has been synthesized as a black microcrystalline powder from C(60) and [Cr(C(7)H(8))(2)] in toluene. [Cr(C(7)H(8))(2)]C(60) is an ionic compound in which the fullerene is negatively charged and the bis(toluene)chromium molecule positively charged. At T = 250 K a reversible first-order phase transition from a primitive cubic high-temperature phase to a triclinic low-temperature phase occurs. The high-temperature phase [Pm3 macro m, a = 9.9840 (1) A, T = 295 K] is composed of dynamically disordered fulleride anions and bis(toluene)chromium(I) cations in a CsCl-type arrangement. The triclinic low-temperature modification [P1 macro, a = 13.6414 (8), b = 13.8338 (7), c = 13.8548 (7) A, alpha = 91.830 (3), beta = 116.776 (2), gamma = 119.333 (2) degrees, T = 235 K] consists of ordered C(60) dimers and two crystallographically distinct bis(toluene)chromium entities.