ELECTRON DENSITY STUDIES ON TETRAAQUABIS (HYDORGENMALEATE)IRON(II). Isolda M. de C. Mendes & Nelson G. Fernandes. Department of Chemistry, Federal University of Minas Gerais, CP702-31270 901- Belo Horizonte, Brazil

The non-spherical distribution of the valence electrons in a crystal of Fe(^{C₄H₃O₄).4H₂O was investigated according to the Hirshfeld deformation refinement, as part of a study of short hydrogen bonds in compounds of divalent transition metals with maleic acid.}

X-ray data were collected on a Siemens four-circle diffractometer at 298 K, by the -2 scan technique, using MoK radiation up to sin/=1.20 Å^-1. For a P space group, 8131 reflections were measured which resulted in 6461 unique reflections (Rint= 0.0085), all of them were considered as observed in subsequent refinements, based on F² with applied extintion corretion. For deformation refinement, 260 parameters, R=0.0299, wR=0.0277 against the conventional refinement, 126 parameters, R=0.0495, wR=0.0474.

In the maleate ligand the two carbonyl groups are linked by a short quasi-linear hydrogen bond with an O...O' distance of 2.428(1) Å. The asymetric crystallographic environment on this bond is revealed by the different O...H distances and electron density maxima of 1.09(2) Å and 0.2 eÅ^-3 for the shorter and 1.35(2) Å and 0.1 eÅ^-3 for the longer bond. Both maxima are at a distance of ~0.7 Å from the oxygen atoms. Density maps show that one of C...O bonds is significantly shorter and with an electron density maximum twice higher than any other of these bonds. Six oxygens atoms form a octahedron around iron atom, the assumed local symmetry is 4/mmm. In this case, the maps reveal an aspherical density distribution with maxima of ~0.2 eÅ^-3 and ~0.3 eÅ^-3 directed between the vertices of the octahedron.