The crystal structure of a cubic modification of tetracyanoethylene has been studied. The space group is Im3 and the cell edge is 9.736 (5) .~. The bond lengths agree well with those in the monoclinic modification, but there are significant differences between the bond angles in the two forms. The central C=C bond is 1.344 (2) • which is not significantly different from the corresponding bond length in ethylene and considerably shorter than predicted from INDO calculations. A new double-atom refinement method has been applied which, from the X-ray data alone, corrects for the apparent shortening of the C=N bond length, as previously found by comparison of X-ray and neutron diffraction data. The corrected value for C=N (1.166 (2)/~) is close to the electron diffraction result on tetracyanoethylene. The molecular packing in the crystal is such that large cavities surround the positions (0,0,0) and (½, ½, ½).

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