X-ray structure analysis of cubic tetracyanoethylene and the length of the C[triple-bond] N bond. Application of a double-atom refinement method

Abstract

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).

Hexamethylbenzene-tetracyanoethylene (1:1) complex

“…The dimensions of the HMB residue are normal and may be compared with those of the free HMB (Brockway & Robertson, 1939) and of the HMB's in the complexes with chloranil (Harding & Wallwork, 1955;Jones & Marsh, 1962) and 1,2,4,5-tetracyanobenzene (Niimura, Ohashi & Saito, 1968). On the other hand, in the TCNE(A) residue the bond distances and angles are somewhat different from the average values found in the free (Bekoe & Trueblood, 1960;Little, Pautler & Coppens, 1971) and complexed TCNE's (Williams & Wallwork, 1967;Ikemoto & Kuroda, 1968;Ikemoto, Yakushi & Kuroda, 1970;Bernstein & Trueblood, 1971); the discrepancies are, however, not fully significant owing to the rather large e.s.d.'s. The TCNE(B) residue will not be discussed because of the imprecise determination.…”

Polymorphism in the [(TTF)2(TCNE)2] Charge-Transfer Complex

“…The nodal properties of the LUMO orbital (b2g symmetry in point group D2h ) of neutral TCNE give a 7r-bonding character to the C-CN bond and an antibonding character to C = C and C-=N bonds. Consistent with this view, the present value of the C=C bond of 1.397 À is higher than that found for cubic [ 1.344(3 ) Â] [24] and monoclinic [1.348(2) À] [25] neutral TCNE, indicating a partial negative charge on TCNE.…”

Structure of the 1/1 complexes of [2.2.2]paracyclophanetriene and [2.2]paracyclophane with tetracyanoethylene

“…For (II), a final refinement was performed with atoms of the cyclophane and TCNE1 (predominant molecule) treated independently with anisotropic thermal parameters while TCNE2 was kept fixed. For both compounds, TCNE: planar model calculated from TCNE crystal structure (Little, Pautler & Coppens, 1971). A coordinate system S is attached to the molecular group and is defined in the following way (see formula): origin, centre of symmetry; x axis, C--C double bond; xy plane, molecular plane of TCNE.…”