METAL-ION COORDINATION IN COBALT FORMATE DIHYDRATE

The structure of cobalt formate dihydrate, Co(HCO2)2 . 2H2O, was deter mined using single-crystal X-ray diffraction data. The crystals are mo noclinic, space group P2(1)/c, with unit-cell dimensions a = 8.680(2), b = 7.160(2), c = 9.272(2) angstrom, beta = 97.43(2)-degrees, V = 571 .4(3) angstrom3, Z = 4. R(obs) = 0.038 for 1282 unique reflections wit h I > 3sigma(I). The crystal structure is found to be isomorphous with those of other divalent metal formates. This structure is interesting crystallographically because the Patterson map is homometric with res pect to the positions of the heavy atoms. The asymmetric unit consists of two independent cobalt atoms on special positions, two formate ion s (HCOO-), and two water molecules. The two cobalt atoms are each coor dinated to six oxygen atoms in an octahedral arrangement. One of the c obalt octahedra contains only oxygen atoms from six formate ions. The second cobalt ion is surrounded by four water molecules and an oxygen atom from each of two formate ions. The two different octahedra are br idged by one of the formate ions and by hydrogen bonds. This network e xtends in a three-dimensional polymeric manner throughout the crystal structure. Each of the four oxygen atoms in the two independent format e ions forms a hydrogen bond to water and is coordinated to a metal io n. It is found that the metal ions lie in the plane of the formate car boxyl group to which they are coordinated, while molecules to which th e formate ion is hydrogen bonded lie more out of this plane.