Structure of heptaaquacalcium salt of the polymeric hexabromodicadmate(II) anion

Abstract

[Ca(H20)7][Cd2Br6] , M r = 8 7 0 . 4 4 , monoclinic, P2~/c {Hall symbol P2~c [Acta Cryst. (1981), A37, 517-525]}, a = 8 . 0 7 9 ( 2 ) , b = 1 8 . 6 7 7 ( 6 ) , c = 12.176 (5) A, f l = 96.60 (3) °, v = 1825.08 A 3, z = 4 , D x = 3.17 Mg m -3, Mo K a radiation, 2 = 0.7107 A, /a = 9.95 mm -~, F(000) = 1584, T = 198 K, R = 0.065 for 2259 reflections. The sample was prepared from a saturated solution containing a 1:1.5 molar mixture Ca of CaBr 2 and CdBr 2. When the sealed container was Ca(l) opened to the air the crystals which had originally Cd(2) formed dissolved and colourless crystals of the title Cd(3) Br(l) compound appeared. The structure consists of discrete Br(2) Ca(H20) 2+ ions ( C a O = 2 .36-2 .47 A) hydrogen Br(3) Br(4) bonded to an infinite Cd2Br 2anion extended along the Br(5) a axis. One Cd is octahedraUy coordinated ( C d Br(6) B r = 2 .75-2 .81 A). It shares edges with two crystalo(1) 0(2) lographica l ly equivalent CdBr 6 octahedra and shares o(3) corners with two CdBr 4 tetrahedra ( C d B r = 2 .53 0(4) 2.61 A). 0(5) o(6) 0(7) Experimental. A crystal 0.5 x 0.3 x 0.3 mm sealed in a thin-walled quartz capillary, Nicolet P2~ diffractometer equipped with a Nicolet LT1 cooling device. Unit cell from 15 strong reflections in the range 20 < 0 < 30 °. No absorption correction was used. Intensities for 2665 reflections in the range 8 < h < 8, 0 < k < 20, 0 < l < 13 (20 < 45°). The standard reflections (~,15 and 272) showed non-systematic variations of about 1%. Equivalent reflections were averaged (R = 0.021) to give 2259 unique reflections. Structure by direct methods (MULTAN80, Main et al., 1980), scale factor, weighting factor (k) and all variable (non-hydrogen) positional and anisotropic displacement parameters refined with SHELX76 (Sheldrick, 1976), w = [o'2(F) + kFo2] -1/2 [where k = 0.0035 and tr(F) is the error derived from counting statistics] to give R = 0 . 0 6 5 , wR = 0 0 7 6 and a goodness of fit of S = 1.02. The average sh i f t /e . s .d .=0 .01 ( m a x . = 0.03). Hydrogen atoms could not be located on the final difference map where the largest feature was 2-13 and the smallest --1.67 e A -3. Atomic scattering factors from International Tables for X-ray Crystallography (1974). Atomic coordinates are given in Table 1.* The * Lists of structure factors and anisotropic thermal parameters have been deposited with the British Library Lending Division as Supplementary Publication No. SUP 42812 (13 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH 1 2HU, England. principal interatomic distances are listed in Table 2 and the structure is illustrated in Fig. 1. Table 1. Atomic positions (x l04 ) and equivalent isotropic displacement parameters (A2 x 104)