2104 Aeta Cryst. (1973). B29, 2104 The Crystal Structures of KICI2 and KIC12.HzO BY STUART SOLED AND GENE B. CARPENTER Metcalf Chemical Laboratories, Brown University, Providence, Rhode Island 02912, U.S.A. (Received 29 March 1973; accepted 14 May 1973) The crystal structures of potassium dichloroiodide and potassium dichloroiodide monohydrate have been determined by X-ray diffraction. Both structures were refined from three-dimentional diffractom- eter data by least-squares methods to give R= 5.7 % and R= 6.0 % respectively. KICI2 crystallizes in space group P2t/c with a=8.507(5), b=10.907(5), c=12.126(5) A, and fl=107.82(4)°; Z=8. KICI2. H20 crystallizes in space group P21/m with a = 8.022 (5), b = 9.611 (5), c= 4"354 (3) A, and fl= 97-03 (4)°; Z= 2. In the anhydrous compound the two independent dichloroiodide ions are nearly linear and symmetric with average I-CI bond lengths of 2.55 ,~. In the hydrate the dichloroiodide ion is linear and symmetric by virtue of lying across a center of symmetry and has an I-C1 bond length of 2.55 *. Each water molecule is weakly hydrogen-bonded to two chlorine atoms, each 3"39 ,~ from the oxygen atom. Introduction Trihalide ions are sometimes linked in the crystal by weak multicenter bonds, as in Csl2Br (Carpenter, 1966). It is of interest to examine the relative roles of multicenter bonding and hydrogen bonding in crystals where both are possible. The structures of only two hydrated polyhalide compounds have been reported, that of KICI4. HzO (Elema, de Boer, & Vos, 1963) and that of HICIa.4H20 (Bateman & Bateman, 1972). Both exhibit weak hydrogen bonding (O-H. • • CI) and possible indications of bonding between chlorine atoms. A more direct comparison of multicenter bonds and hydrogen bonds should be afforded by the struc- tures of the hydrated and anhydrous versions of the same compound. For this reason we chose to investi- gate KICI2 and KICI2. H20. The preparation of 'potassium dichloroiodide' was first described by Wells, Wheeler & Penfield (1892). In the course of nuclear quadrupole resonance studies on the ICI~- ion (Cornwell & Yamasaki, 1957; Yamasaki & Cornwell, 1959), it became clear that both hydrated and anhydrous forms of KIC12 exist. In both cases the data led the authors to propose for the ICI~- ion a bonding scheme based on p atomic orbitals. At about the same time Allison & Cheesman (1958) also showed, by physical and powder diffraction methods, that both KIC12 and KICI2. H20 exist. Experimental Sample preparation Anhydrous KIC12 was prepared by the method de- scribed by Wells (1901) for the preparation of CslC1,; crystals formed upon cooling a hot aqueous solution of KNOa, 12, and HCI. The crystals were initially thought to be hydrated because they formed in aqueous solution and because Cremer & Duncan (1931) were able to obtain the anhydrous compound (only as a powder) by a strictly dry procedure. The true nature of our crystals was revealed by the structure determina- tion. Crystals ofKICl2. H20 were prepared according to the original methods of Wells, Wheeler & Penfield (1892) by bubbling chlorine through an aqueous solution of KCI and I2. Both compounds tended to lose iodine on standing, but decomposition could be prevented by coating them with a halocarbon grease (Kel-F). Unit cell and space group Crystallographic data for the two compounds are set out below. KICI2 KICI2. H20 System Monoclinic Monoclinic a (A,) 8.507 (5) 8.022 (5) b 10.907 (5) 9.611 (5) c 12.126 (5) 4.354 (3) fl (°) 107.82 (4) 97.03 (4) a:b:c (X-ray) 0.7800:1 : 1.1118 0.8347:1:0-4530 a:b: c (optical) -- 0.8319: 1:0.4544 Absences hOl, l= 2n + 1 0k0, k = 2n + 1 0k0, k = 2n + 1 Space group P2x/c P21/m Z 8 2 d, (gcm -3) 2.91 2.53 d,, 2.87 (5) 2.44 (5) 62-0 parallelpiped bounded by {001), {110) /z(cm-1), Mo Kc~ 75.3 Habit parallelpiped bounded by {100), {010), {001 }, {110} Unit-cell and space-group data were obtained from preliminary Weissenberg and precession photographs and from 15 20 values carefully measured with a Picker four-circle diffractometer. The optical axial ratios for the hydrate were measured by Penfield (Wells, Wheeler & Penfield, 1892). Systematic absences determined the space group of K1Clz uniquely. For the hydrate the