Characterization of the Solid-Solid Phase Transition of Co2(CO)6(AsPh3)2

The dimeric Co2(CO)6(AsPh3)2molecule contains an unsupported Co-Co bond, axial arsines, and equatorial carbonyls. At room temperature, the molecule crystallizes in the R3̄ space group [a = 15.73(3) Å14.240- (2) Å] packing in chains along c which are hexagonally arranged in the ab plane. When the temperature is lowered, no significant changes are found in the molecular structure until a second-order phase transition [Tc= 206(3) K] occurs by doubling the c axis [at T = 123 K a = 15.233(4) Å, c = 28.440(6) Å, space group R3̄]. The major molecular conformational changes related to the phase transition are the rotations of Co(CO)3and AsPh3fragments about the 3-fold axes and those of the phenyls about their As-C bond. X-ray studies at 16 different temperatures have been performed in order to describe the details of the small continuous structural changes that occur in the low-temperature phase.