The crystal structure of iodine at 110°K has been refined. The I-I bond, 2.715/1,, e.s.d. 0-006/1,, appears to be significantly longer than the bond, 2.662 A, in gaseous iodine molecules.

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The crystal structure of (PNC12) 4 has been refined by three-dimensional Fourier syntheses. The crystallographically non-equivalent P-I~ bonds in the puckered eight-membered ring have an equal length of 1.570 A, s.d. 0.01 A. The average value for the P-C1 bond lengths is 1.989/~, s.d. 0.004 A. The configuration of the molecule is compatible with Craig's or Dewar's theory in which a delocalization of the p-z and d-z electrons is assumed. IntroductionIn an early structure determination of the metastable modification (Lund, Paddock et al., 1960) of (PNC12)4 by Ketelaar et al. (1939), the lengths of the P-N bonds in the eight-membered ring were found to be equal within rather large experimental error. However, a theoretical prediction (Craig, 1959) of equal S-N bond lengths in the analogous case of (NSF)4 proved to be incorrect (Wiegers & Vos, 1961). Therefore it seemed desirable to refine the structure of (PNC12)4 by three-dimensional methods in order to obtain more conclusive evidence about the P-N bond lengths in this compound.In connection with our work on the thiazylhalides, an investigation of some more phosphonitrilic compounds is in progress. A preliminary structure determination of the stable form of (PNC12)4 by twodimensional syntheses has already shown that the (PNC12)4 molecules in this modification are centrosymmetrical. In addition the crystal structures of (PNCIg)6, [PN(0H) 914.2H20 and [PN(0H)(0Na)]3. 4H20 are being investigated. The above structures will be refined by three-dimensional methods. ExperimentalThe substance was prepared by the method of Schenk & RSmer, described by Ketelaar (1939). Crystals were obtained from a solution in benzene. For the intensity measurements cylindrical crystals were used; they were made by touching a rotating crystal lightly with filter paper soaked with benzene. The multiple-film technique was applied, IlfordIndustrial G, Industrial B and Industrial C film being used. Corrections for absorption were calculated from Bond's table (Bond, 1959). /,By comparing the [Fr-values of the reflexions from crystal 1 with those of corresponding reflexions from crystal 3, we found that the intensities obtained from crystal 1 showed strong extinction. These extinction errors could greatly be reduced by comparing, for each layer line about the c axis, the intensities 11 of the medium and strong reflexions from crystal 1 with 540 REFINEMENT OF THE STRUCTURE OF METASTABLE (PNCI2) 4 the intensities I~ of the reflexions from crystal 2.A plot of I2/I~ versus I1 for the zero layer line is shown in Fig. 1. It is seen that the correction factor in the intensities /1 amounts to about 3 for the strongest reflexions Mc0. The extinction effect appeared to decrease rapidly for increasing values of l, and is vanishingly small for the reflexions hk3. For 8 strong reflexions no reliable correction for extinction was obtained.During the refinement 623 independent reflexions with reliable intensity were taken into account. RefinementThe approximate structure is described by Ketelaar (1939). Th...

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