By use of X-ray diffractometer intensity data, the crystal structure of hexachlorocyclotriphosphazene, N3P,CIQ, has been refined to R 0.048 for 1222 reflexions. The phosphazene ring is almost planar but there are small deviations from planarity of up to 0.04 A. The P-N bonds are all equal in length, 1.581 A (u 0.004-0.005 A) and the P-CI bond lengths range from 1.991-1 *995 A (u 0 ~0 0 4 -0 ~0 0 5 A). The mean N-P-N angle in the ring (1 18.4") is significantly smaller than the mean P-N-P angle (1 21.4"). There are only slight deviations from the ideal molecular symmetry DSh (6m2). An analysis of the atomic thermal vibrations shows that at 25' f 2 "C the molecule is undergoing translational vibrations with root-mean-square amplitudes of 0.1 7-0.1 9 A combined with angular oscillations about the molecular axes whose root-mean-square amplitudes are 3.4-3.6".

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Crystals of tetrameric phosphonitrilic dimethylamide, P,N,(NMe,),, are tetragonal, a = 13-00, c = 8.59 A, space group I 4 with two molecules in the unit cell, molecular symmetry I (S,). The atomic positions have been determined by Fourier and least-squares refinement of X-ray diffraction data. The molecule has a puckered eight-membered ring of alternate phosphorus and nitrogen atoms, P-N bond length 1-58 A. The exocyclic P-N bonds are 1-67 and 1.69 long, considerably shorter than the accepted P-N single-bond length (1.77 A). It is suggested that this shortening is due to (i) the phosphorus atom's being in the sfis-hybrid state and forming pn-dxbonds with the cyclic nitrogen, and (ii) a tendency of the exocyclic P-N bond to double-bond character as a result of donation of electrons from the lone pair of the exocyclic nitrogen. The more important valency angles are: N-P-N (cyclic) 120°, P-N-P (cyclic) 133O, N-P-N (exocyclic) 104".ALTHOUGH the phosphonitrilic halides have been known for many years there has until recently been comparatively little interest in them and their derivatives, and very few of their crystal structures have been examined. Craig's proposal that these compounds are aromatic in character and that aromaticity is not, as with carbon systems, confined to six-membered rings, has created a need for knowledge of their precise stereochemistry .Until two years ago the only detailed crystal structure determination of a phosphonitrilic compound was that of (PNCl,), by Ketelaar and de Vries.2 This has recently been refined by Hazekamp et aL3 and in addition there are X-ray diffraction studies of (PNCI,), (Wilson

cml, attributed t o the asymmetric stretching frequency of a cumulative BfN=C ~y s t e m . ~ By contrast a B-X=C system would be expected6 to have v(C=N) in the region of 1590-1690 cm-l. Higher stretching frequencies are characteristic of cumulative ri-systems [e.g. v,(C=C=C) a t 1920-2000,7 or v,,(C=N=C) at ca. 1845 cm-l (ref. S)]. Molecule (11) offers the possibility of a high B-N bond-order because the donor and acceptor abilities of the nitrogen and boron atoms are confined to one bond.

Intensities of (hO1), (h/CO) and (Mcl) reflexions were estimated visually from Weissenberg photographs taken using MoK radiation. To obtain a range of intensity for each reflexion, instead of the usual method of interleaving films with metal foil, films of different speeds (Ilford Industrial G and B) were used (Iball, 1954). Reflexions were observed to a sin 0 limit of 0.59. Roughly equidimensional crystals of side 0.3 mm. were used and, within this range of sin 0,