Synthesis, Crystal Structure and Magnetic Property of Disodium Biscarbonato cobaltate（Ⅱ） Tetrahydrate, Na2Co（CO3）2·4H2O

The title compound, Na 2Co（CO 3） 2·4H 2O, was synthesized by dropwise addition of 1.0 mL of 1 mol/L Co（CH 3COO） 2 to 50 mL of aqueous solution containing 7.0 g（66.0 mmol） of Na 2CO 3 and 2.5 g（29.8 mmol） of NaHCO 3 under stirring. A rose colored rhombohedral crystal grew by the slow evaporation of the solution of the title compoud at room temperature. The crystal structure was established on the basis of the single crystal X ray diffraction data. Na 2Co（CO 3） 2·4H 2O crystallizes in the monoclinic space group C2/c with a =0.856 6（1） nm, b =0.714 6（1） nm, c = 1 438 7（2） nm, β =97.59（1）° , V =0.873 0（2） nm 3 , D x=2.260 g/cm 3, F （000）=596, M r=296 99, μ （Mo K α）=21.06 cm -1 , Z =4 and the final R（F） =0.032 and wR（F 2） =0.068 on the basis of 947 reflections （ F 2 ≥ 2σ（F 0 2））. The crystal consists of H 2O molecules, Na + ions and 2D 2 ∞[Co（CO 3） 2] 2- layers extending parallel to （001）. H 2O molecules and Na + cations are located between the 2D layers. Each Co atom is tetrahedrally coordinated by four O atoms from four CO 2- 3 groups with d （Co\O）=0.206 5（2） nm, 0.207 6（2） nm and the angle of O\Co\O is in the range of 97.7（1）°—128.5（1）°. The CO 2- 3 group bridging two Co atoms via two O atoms deviates from D 3h symmetry with d （C\O）=0.127 2（3）\0.129 9（3） nm and the angle of O\C\O is in the range of 119.5（2）°\123.4（2）°. Each Na atom, with two close Na neighbours at 0.338 4（2） nm and 0 346 7（2） nm, respectively, is octahedrally coordinated by six O atoms from four H 2O molecules and two carbonate groups belonging to two layers with d （Na\O）=0.232 7（2）\0.241 1（2） nm. Of two crystallographically different H 2O molecules, one forms hydrogen bonds to two CO 3 2- groups of one 2D layer, and the other to those from two different 2D layers. Na 2Co（CO 3） 2·4H 2O is antiferromagnetic below 16 K, and between 30\300 K its magnetic behaviour obeys Curie Weiss law χ m（cm 3·mol -1 ）=2.625/（ T -12.1） with μ eff =4.5 B.M. at 298 K.