The Structure of Hexaamminechromium(III) Pentachlorocuprate(II), [Cr(NH₃)₆][CuCl₅]

The crystal structure of hexaamminechromium(III) pentachlorocuprate(II), [Cr(NH₃)₆] [CuCl₅], has been redetermined from three-dimensional X-ray data collected by counter methods. The material crystallizes in space group Fd3c of the cubic system with 32 formula units in a cell 22.240 (7) Å on an edge. The calculated density of 1.908 g/cm³ agrees well with that of 1.892 g/cm³ measured previously. The structure has been refined by least-squares methods to a final R factor on F of 3.7% for the 633 reflections above background. The positions of the hydrogen atoms of the ammine groups were included in this refinement. In addition, the charges on the atoms were determined. The Cr(NH₃)₆³⁺ ion is required crystallographically to have 5 site symmetry. The coordination geometry is octahedral. The N-Cr-N angle is 89.65 (12)° and the Cr-N bond length is 2.0644 (25) Å. The [CuCl₆]^3- ion is required crystallographically to have 32 site symmetry and is therefore a regular trigoaal bipyramid. There are two independent Cu-Cl distances, one along the threefold axis and one in the equatorial plane. The Cu-Cl bond lengths, uncorrected for the effects of thermal motion, are Cu-Cl_ax = 2.2964 (12), Cu-Cl_eq = 2.3912 (13) Å. This significant shortening of the axial bond lengths in a trigonal-bipyramidal MX₅ complex is compared with results available for other MX₅ complexes. This shortening may be explained by the stereochemical activity of the 3d electrons.