The structure of octachloropentafulvalene

The structure of octachloropentafulvalene has been redetermined by a 3-dimensional X-ray crystallographic analysis. The space group is monoclinic, C2/c, and cell parameters are a = 14·998, b= 7·9110, c= 11·8068 Å, β= 103·38°. The X-ray intensity data were measured on a computer-controlled diffractometer using Mo radiation. The least-squares structure refinement used anisotropic temperature factors for C and Cl and gave a final R of 0·036. Corrections were applied for absorption, Cl dispersion and secondary isotropic extinction. The central CC is twisted by 37° as a result of Cl?Cl repulsions from one C₅Cl₄ half to the other. The central CC length, 1·365 Å, is only slightly larger than the standard 1·34 Å ethylenic value. The bond distance increase can be explained in terms of a decrease in π-bond character accompanying the CC twist. Carbon-carbon distances in the 5-ring have typical values for the cyclopentadienylidene moiety; there is no evidence for single-double bond delocalization.