CRYSTAL STRUCTURE OF SELENIUM DITHIOCYANXTE
May 20, 1934
[CONTRIBU?ION FROM
THE SCHOOL OF
CHEMISTRY, RUTCERSUNIVERSITYj
The Crystal Structure of Selenium Dithiocyanate BY STANLEY 11. OHLBERG AND PHILIP A. VAUGHAN RECEIVED OCTOBER12, 1953
A,
A.
Selenium dithiocyanate, Se(SCN)s, belongs t o the orthorhombic system with a = 9.87 A,,b = 13.03 c = 1.44 and Theospace group is DhE-Pnma and tbe molecules belong t o point group Cs-m. The molecular parameters are Se-S = 2.21 h., S-C = 1.69 A.,*C-N = 1.13 A , . LS-Se-S = 10lo,LSe-S-C = 1U4” and the dihedral angle between the S-Se-S and Se-S-C planes is 79 . Z = 4.
Ohlberg and van der Meulen’ have recently described the preparation and some of the properties of selenium dithiocyanate. The two most probxble configurations for this compound are Se(SCN)2 and Se(NCS)2. The present lack of information concerning the structure of the thiocyanate group provides additional incentive for determining the structures of this compound. Experimental Unit Cell and Space Group.-Crystals of Se(SCS)* were ribtained from aqueous or dioxane-ether solutions as small prisms. Weissenberg photographs about the c (prism) :iuis showed t h a t the compound is orthorhombic. Systematic absences observed on (hkO), ( h k l ) and ( O k l ) WeissenIierg photographs indicate t h a t the space group is either CHV-Pn2a or Dj//-Pnma. T h a t the latter is correct is cvitlent from interpretation of Patterson projections. The cell parameters were determined fro? indexed oscillation photographs. They are a = 9.87 A . , b = 13.03 A. and c = 4.44 A. The density calculated on the basis of four molecules per unit cell is 2.273 g./cc., which is in agreement with the density of 2.265 g./cc. determined by the flotation method. Intensities.-The intensities of the (hkO) and (OR!) reflections obtainable with Cu Kcu radiation were determined by visual comparison on triple Weissenberg photographs and converted t o values of I F / 2by division by the Lorentz and polarization factors. No corrections were made for absorption errors. However, very small crystals were used for the intensity measurements and we were able t o obtain satisfactory agreement between observed and calculated data.
Structure Determination The locations of the selenium and sulfur atoms were obtained from Patterson projections on (001) and (100). These projections were easily interpreted on the basis of four Se(SCN)2 molecules, the Se atoms of which are in the mirror planes a t y = and y = 2 of the space group D i g P n m a . The atomic coordinates? are 4 Se in 4(c) : 3 , 2, z; 2, 2, 8; - x, 2, 3 z; 3 x,$, 3 - z; 8 S,C and N in S(d) f l y , Z; 3 X, 3 - y) 2 - Z; L , 4 y,8; 3 -
a
+
+
TABLE I REFINEMEXT OF PARAMETERS Parameter
Patterson
XSe
103
-0.050 ,534 ,059 ,117 ,281
xc
.....
YC
, . . . .
ZC
,....
XN YN JN
, . . . .
25 e
XS
YY
First Fourier
- 0.045 ,513 ,059 ,119 ,294
. . . . .
,196 ,108 ,522 287 100
.....
,
Big
Final
-0.u37 500 058 110 285 ,206 ,111
171 305 1(!,5 600
to calculate Fourier projections on (001) and ( I O ( ) . The C and N positions were not apparent from thcse
N
ta Fig. 1.-Fourier projection of Se(?CS), on (001). Solid lines are at intervals of 4 e l e ~ t r o n s / A . ~dashed ; lines are a t 3 and 5 e l e c t r ~ n s / A . ~dots ; are at positions corresponding to the final parameters.
+
+ + 3, + :E, 7 , 8; 3 - x,3 + y , 3 + x, 3 - y, z; 3 + x,y, 3 - z. Parameters for Se and S obtained from the Pat-
XI
2
2,’
2;
terson projections are given in Table I. Since the Se-S and Se-Se maxima on the Patterson projection on (100) were unresolved multiple peaks, the z coordinates were estimated from peak shapes and assignment of 2.21 A. to the Se-S d i ~ t a n c e . ~ Structure factors were calculated from the Se and S coordinates and the signs of these were used
S
I
( 1 ) S hI. Ohlherg and P. A
van der nfeulen, THISJ O I J R N ~ I , , 76,
997 (1933). ( 2 ) “International Tables for X-Ray Crystallography,” Vul. I , the Kynoch Press, Birmingham, England, 1952, 11. 1.51. 131 . . I*. Paulini.. ”Nature uf t h e Chemical Bond.” .~ Cornel1 Universitv Press, Ithnca, N.Y . , l Y 4 5 , p 164. I
~
C
Fig. 2.-Fourier projection of Se(SCK)2 on (100). Solitl lines are at intervals of 4 e l e c t r o n ~ / A . ~the ; lowest solid line is a t 8 e l e ~ t r o n s / A . ~the ; dashed line is a t t i ~ l e c t r o n \ / ~ . ~ ~ dots are a t positions corresponding t o the fiiial parameters.
2630
STANLEY
>I. OIiLBERG
AND P H I L I P
projections and projectioiis were made using Fobsd Fcalc,j for the Fourier coefficients. -1Laxima corresponding to C and N appeared on these although the two atoms were unresolved in the (100) projection. The z parameters were calculated by making the assumption that C-N distance is 1.15 k . The new parameters, which are given in Table I, were used to calculate structure factors, the signs of which were used to compute the final Fourier projections. These are shown in Figs. 1 and 2. All of the atoms are clearly resolved on the (001) projection. On the (100) projection two sulfur atoms are unresolved. To obtain the final z parameters for sulfur the least squares4 method was used. The observational equations were of the form
d E G i In 1
,#I I FO
=
v%G { - AK
+
- A B (sin Q , ~ ~ ~ / A I * Azs(l/I F L ! FcI Ids,)
1
where eK is the scale factor and e-B(sln is the temperature factor. The temperature factor was assumed to be isotropic since the resolved peaks are nearly isotropic in both projections and the temperature iactors for the observed (hkO) and (Okl) reflections are nearly the same ( B = 3.82 and 4.06, respectively). Solution of the normal equations gave Ik7 = 0.0103,AB = 0.27 and AZS = 0.0006. The change in zs is negligible; it amounts to only 0.002 I A. The C and S atoms are also unresolved in the (100) projection. To calculate final z parameters for C and N we assumed that the C-N maximum is at the center of the C-N bond and that the SCN group is linear. The latter assumption is consistent with the (001) projection. The final atomic coordinates are given in Table I and a drawing of the structure on Fig. :%.
-4. V"iUG€iAN
Vol. 7 6
TABLE I1 OBSERVED A V D CALCULATED STRUCXRE I'ACTCIKS (kk0) Reflections hkl
'FOI
200 400 ROO 800 10 0 12 0 210 410 tiin 810 10 1 12 1 020 220 420 620 820 10 2 12 2 230 430
117 72 20 62 1)
'70
0
20 20 55 77 .i8
0 0
0 0
670
am 10 3 . 0 12 3 0 040 240 440 640 840 10 4 . 0 12 4 . 0 250 460 650 8.50 10 5 . 0 12 5.0
om ?6(3 460
660 020 040
23 I6 81 198 66
