The crystal structure of C7,.6(Ss) at 100 K shows C70 to be ordered dnd the S , molecules to be partially disordered The symmetry of C,, is D,, to within experimental accuracy Crystal structures of several phases containing unsubstituted C,, have been determined by single(or twinned)-crystal X-ray diffraction: the cubic low-temperature phase of C,, (at 11 0, 153, and 200 K) [ 11, the cubic room-temperature phase of C,, [2], several phases of C,, co-crystallized with organic molecule: C,. 3 (C,H,O,) [3], C6,.2(C,,H,S,) (at 150 K) [4], C,,~CS,.(C,,H,,S,,) [5], C,;C,H,~CH,I, [6], and C,,.4(C6H6) (at 173 [7] and 104 K [S]
). In all of these structures, C,, is disordered or shows very large amplitudes of motion. Here, we describe a structure determination of the phase C7,.6(S,) at 100 K with ordered C,, molecules.
The results of a previous study of this compound at room temperature [9] were unsatisfactory for several reasons: I ) Some chemically equiuafent C-C bond lengths, with e.s.d.'s quoted as 0.03 A, differed by more than 0.2 A which is chemically very unlikely. The differences between chemically inequivalent distances expected on the basis of quantum chemical calculations is only -0.12 A [lo]. Hence, the experiment [9] cannot distinguish reliably between inequivalent C-C bonds. 2 ) If the refined distagces of chemically equivalent bonds are averaged they agree well with theory [9] [lo]. It has not been tested, however, whether a model consisting of C,, molecules with averaged bond lengths accounts for the diffraction data as well as the one which is not averaged. 3) The resolution of the diffraction data was limited to -1 A and the R values seemed rather high for an ordered structure ( R ( ( F ( ) = 6.5%, wR(IF21 = 7.1 %). Thin platelets of C7,.6 (S,) with well developed (100) faces were obtained, as described in 191, by slow evaporation of a CS, solution of C,, and S, in relative amounts of 1 :6').
') Crystal Data. Dark platelets of C70.6(S,), orthorhombic, space group Amm2 with a = 37.953, 6 = 20.241, c = 10.226 8, at 100 K, 2 = 4, graphite monochromatized MoK, radiation (A = 0.71069 A). Scan angle and counter aperture for the diffraction experiment were carefully chosen in order to minimize possible measurement errors resulting from partially overlapping reflection scans, the unit cell distance a being exceptionally long for a small molecule structure. Least-squares refinement of 4363 reflections with F > 60 (F), 409 parameters, R = 3.73%, W R = 3.82% for the constrained and restrained model described in the text. Atomic coordinates, bond lengths and angles, and thermal parameters have been deposited at the Cumbridge Crvstai- logruphic Data Center.