Metal ion coordination in cobalt formate dihydrate

Abstract

The structure of cobalt formate dihydrate, Co(HCO₂)₂ · 2H₂O, was determined using single-crystal X-ray diffraction data. The crystals are monoclinic, space groupP2₁/c, with unit-cell dimensionsa=8.680(2),b=7.160(2),c=9.272(2) Å,β=97.43(2)°,V=571.4(3) ų Z=4.R _obs=0.038 for 1282 unique reflections withI>3σ(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 respect to the positions of the heavy atoms. The asymmetric unit consists of two independent cobalt atoms on special positions, two formate ions (HCOO⁻), and two water molecules. The two cobalt atoms are each coordinated to six oxygen atoms in an octahedral arrangement. One of the cobalt 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 bridged by one of the formate ions and by hydrogen bonds. This network extends in a three-dimensional polymeric manner throughout the crystal structure. Each of the four oxygen atoms in the two independent formate ions forms a hydrogen bond to water and is coordinated to a metal ion. It is found that the metal ions lie in the plane of the formate carboxyl group to which they are coordinated, while molecules to which the formate ion is hydrogen bonded lie more out of this plane.