The study described in the present communica- be restricted to ;parameters of 0, 4, l / ~and :i i. tion was undertaken as a part of a program involv - Although there were no grounds for eliminating ing crystal structure studies on organoseleniuni Pnam in the earlier considerations, it was deficompounds. The structures of four compounds nitely ruled out later by the Patterson projections of the type K2SeX2,where It is phenyl or p-tolyl 011 ( 100) and I 0 IO). These projections indicated and X is chlorine or bronljnz, have already been differences of 0.140 and 0.360 in the z parameters Since I I ~ of Se and C1 rather than the values 4 or 2 rereported from this Lci11cmtory structures of the type K&eX had been reported quired by the space-group Pnam. The spacein the literature, it was considered of interest to groups is accordingly Pna. undertake the present investigation. Since the projection on (001) is the only centroPreparation of the Crystals. --The triphenyl- symmetric projection o f the space-group Pna, it selenonium chloride used in this study was pre- was decided to prepare a Fourier summation on pared by the method of Leicester and Berg- this face. The selenium atoms are by far the strorn.5 The product was recrystallized twice heaviest &oms in the substance and will accordfrom methyl ethyl ketone, dried at 100' and iden- ingly determine the signs of the majority of the tified as anhydrous triphenylselenonium chloride structure factors. From an inspection of the inthrough volumetric determination of the chloride tensities 11i reflections of the type hO0 and Ok0, present. Crystals suitable for the preparation of it w a 5 possible to assign approximate values of the X-ray diffraction photographs were obtained by .u m t l parameters of the selenium atom. By use the slow evaporation of 4 d u t i o n (,I the sub- d thew values, the signs of all structure factors of stance in a mixture $11equal parts of hx.)lute a l - the type hkO were teiitatively fixed and these were cohol and benzene. The crystals were needle- theu used to prepare the Fourier summation on like, elongated in the tfirwtiotr with priricip11 1001) I'his summation showed, in addition to a large peak due to the selenium atom, a smaller faces of the form (110). The X-Ray Study.- -Sets UI Iotcltloil m d peak doubtless due to chlorine. Using the resultWeissenberg photographs were prepared about ing chlorine parameters, the signs of five structure all three orthorfioinbic c l ~ e 5by use of filtered factctrs were changed, aiid a second and final Fourcopper radiation. For photographs ,ibout the ier sunimtttioti was prepared. This projection is u and h axes, i t was necessary to cleave the crys- shown in Fig. I . It shvuld be pointed out that tals perpendicular to the c a The diiiiensioiis the coiistunt term F(000) was not included in the suniniatioii, with the result that the size of the seof the unit-cell (based o,n C = l..j?Ih A,) were found to be a = 10.93 A., b = 10.38 A., i = 14.51 leniutii peak is disproportionately large. Siiicc Fourier projections on the other two A\. all *0.03 8. The approxirnalc density of the crystals was found to be 1.4 g. cc. hy flotation in tdces oI the unit cell are riot centrosymmetric, mixtures of benzene and carbon tetrachloride. these suiiimations inay be obtained only with conThe density computed froiii the X-ray data on the siderdble tiificuity arid were not attempted. However, Patterson projections on (100) and (010) assumption of 4(C,jH&SeCl per unit-cell is 1.Ys).i g./ cc. The only systematic absences were found 'ire readily carried out and these were accordingly to be h01 with h odd and OK1 with ( R I ) odd, in- prepared. .is is typical of a compound containing dicating the space-group to be either ha-(?:, or one ,itoni considerably heavier than any other, little detail was obtained from the Patterson proPnam-D& jections. 111 addition to the Se-Se peaks which While the space-group Pna places no symmetry occurred a t positions consistent with the x and restrictions on the structure of the (CnH&SeCl parameters of the selenium atom, the only other molecule (or of the (C6H&Se4 ion), the space- peaks were of appropriate height and position to group Pnam requires either a center of symmetry represent Se --C1interactions. From these peaks, or a mirror when only four structural units are it was possible to obtain the difference in the 2 placed in the unit-cell. rhe selenium and chlo- l)aratnetcr\ of the selenium and chlorine atoms. rine atoms, each i n fnurfold positions, would thus Due to the polar character of the c axis, i t is nec( I ) Based in work prrfoiriied under Cuulrdcr Nbuur-275 bctweeo essarv t o fix arbitrarily. the z parameter for one the Office of h-aval Resenirh 9nrI rhc 7-niversity of Cnllfornia, 1 n i ,xtoni
