Structure of tetraaquacobalt(II) bromide dihydrate (cobalt bromide hexahydrate)

Tetraaquacobalt(II) bromide dihydrate, [Co(H2O)4]Br2 x 2H2O, Mr = 326.83, monoclinic, C2/m, a = 11.0158(12), b = 7.1694(10), c = 6.9111(10) A, beta = 124.761(7)degrees, V = 448.4(1) A3, Z = 2, F(000) = 314, Dx = 2.420 g cm-3, lambda(Mo K alpha) = 0.71069 A, T = 296 K, mu = 107.13 cm-1, R = 0.022 for 532 unique reflections having I greater than 0. The single type of Co ion is octahedrally coordinated by four equivalent water-molecule O atoms at the observed distance 2.081(2) A and two equivalent Br ions at the observed distance 2.6048(5) A in an array which, with respect to its angles, departs only slightly from regular octahedral geometry. The cobalt-oxygen-bromine complex was found to manifest rigid-body behavior. The cobalt-oxygen distance corrected for rigid-body motion was found to be 2.088 A; the corrected cobalt-bromine distance, 2.6081 A. Associated with each complex are two water molecules of hydration. Assignment and refinement of the coordinates of the four non-equivalent H atoms permitted detailed analysis of the hydrogen bonding, which involves each of the four non-equivalent H atoms significantly. The structure as determined in this study differs markedly from that presented in the sole published single-crystal study [Stroganov, Andreev, Kozina & Solov'ev (1961). Vestn. Leningr. Univ. Ser. Mat. Fiz. Khim. 16, 114-119]. In particular, the space-group assignment differs and the pattern of bromine-oxygen distances is quite different. Contrary to Stroganov et al., this structure is isomorphic with that of cobalt chloride hexahydrate.