The structure of hexaiodobenzene has been elucidated by an X-ray study based on photographic data. The crystals are monoclinic pseudo-hexagonal, space group P21/n, which here requires the C6I6 unit to be centrosymmetric. Within experimental error the carbon skeleton is a regular planar hexagon with mean C–C distance 1·41 ± 0·05 Å, and the iodine atoms lie very close to this plane; they show small displacements (ca. 0·04 Å) alternately above and below the mean plane of the ring, to give a mean I–I intramolecular distance of 3·507± 0·003 Å. The closest intermolecular distance is similar (3·76 Å), and surprisingly short as compared with twice the van der Waals radius (4·3 Å). The mean C–I distance is 2·093± 0·037 Å. The molecules are stacked parallel to one another to form columns in the unique crystallographic direction, but the planes of the molecules are tilted with respect to the column axis. The whole crystal could be regarded as a dislocated cubic close packing of iodine atoms, with one iodine in seven replaced by a planar hexagonal C6 unit.
