. Can. J. Chem. 58, 1651 (1980). X-ray diffraction studies have revealed that K4U02(C03), is isostructural with (NH,),UO,(CO,),; the crystal is monoclinic with a [Traduit par le journal]

No abstract

The reaction between mercury(II) chloride and thiosemicarbazide (1:2 molar ratio) in ethanol–water (1:1 by volume) gave, almost quantitatively, dichlorobis(thiosemicarbazide)-mercury(II), HgCl2(tsc)2. There was a trace of unidentified black material as a side product. The crystals of HgCl2(tsc)2 are orthorhombic with a = 8.675(7), b = 8.123(6), c = 15.786(11) Å, Z = 4, and space group Pbcn. The HgCl2(tsc)2 molecule in the crystal has a twofold axis and the mercury atom is bonded to two chlorine atoms at 2.841(3) Å, and two sulfur atoms at 2.417(3) Å, in a highly distorted tetrahedron. The bond angles are: S—Hg—S = 160.7(1)°; Cl—Hg—Cl = 96.6(1)°; S—Hg—Cl = 89.5(1)° (twice) and 103.4(1)° (twice). However, if the close contacts, [Formula: see text] were counted, the coordination around the Hg atom would be an approximate octahedron.

Crystals of mercury(II) ethylxanthate, [Hg(C3H 5-OSz)2], obtained from acetone are monoclinic with a = 9.300 (2), b = 6.693 (2), e = 19.585 (9) A, fl = 100.94 (3) °, space group P2~/e, Z = 4. The structure was determined by the heavy-atom method from 2111 diffractometer-measured reflections. The crystal consists of mica-like two-dimensional sheets formed by mutual bridging of Hg n and ethylxanthate ions. The Hg H ion is bonded to four S atoms with Hg-S distances of 2.417 (4), 2.421 (4), 2.789 (4) and 2.854 (4) A. The coordination geometry is a very distorted tetrahedron with a large and a small S-Hg-S angle of 147.7 (1) and 84.3 (1) °, respectively. The sheets are stacked parallel to the (001) plane; alternate layers are related by a center of symmetry. In contrast, the same compound crystallized from CCI 4 belongs to P2 I. The structures within the layers are the same but the packing arrangements of the sheets are different in the two crystals.

946PHASE EXTENSION AND REFINEMENT 1, 5 and 7. It is clear that the use of a higher NMIN gives better results for the average phase deviation as well as a lower number of phases with an error of over 250 mcycles. An E map calculated from the phases obtained with NMIN equal to 7 revealed 162 atoms among the 240 strongest unique peaks; this number is not much larger than in the case of NMIN = 1, but the peaks were much higher, although the number of contributing reflections was smaller.The alternative procedure was also tested using A s = 0 in (6), which is equivalent to the use of the usual formula (5). From the results given in the lower half of Table 3 it can be concluded that: (a) the final phase sets are almost centrosymmetric and (b) the use of a higher number for NMIN only slightly slows down the appearance of the other enantiomorph. For the sake of comparison an E map was calculated from the phases obtained with A 3 ----0 and NMIN = 7. Only 93 out of the 243 strongest unique peaks could be identified as atoms.The total computing time for the tangent extension procedure mentioned above ranged from 78 to 105 c.p.u.s (on a Cyber 73 computer). The procedure based on the graphical determination of new phases took 147 s c.p.u, time for the 400 atom structure despite the fact that only + 1300 unique triplets were used in the final refinement cycles. Our conclusion is that the procedure based on graphical phase determination is rather expensive and difficult to optimize for accepting new phases. Since the procedure using (5) leads to a maximum number of more or less correct phases in a minimum of time, it is to be preferred. If this procedure is combined with a refinement procedure based on (6) the results are sufficiently enantiomorph specific to lead to a correct solution, starting from a medium-sized phase set.The main conclusion of this paper is that with a relatively simple improvement in the estimates used, the centricizing tendency of the tangent formula is efficiently blocked. However, since it is expected that better estimates will improve the quality of the final map, this will be an important part of our future efforts.The authors thank Dr C. H. Stam and Professor B. O. Loopstra for criticizing the manuscript. One of the authors (GJO) is indebted to the Netherlands organization for the advancement of pure research (ZWO) for financial support. Acta Cryst. A31,227-233. SAYRE, D. (1972). Acta Cryst. A28, 210-212. SAYRE, (1978 AbstractAny cubic crystal structure can be divided into small units in the form of congruent semi-regular (Archimedean) truncated octahedra. The centers of these polyhedra can be chosen at invariant equivalent positions for most cubic space groups.