Structure of hashemite, Ba(Cr,S)O4

Raman spectroscopy and structural study of baryte-hashemite solid solution from pyrometamorphic rocks of the Hatrurim Complex, Israel

A number of the baryte, BaSO₄, - hashemite, BaCrO₄, solid solution compounds were synthesized previously. In this study, Raman spectra of naturally occurring phases belonging to the baryte-hashemite series from the pyrometamorphic rocks of the Hatrurim Complex were investigated. The Raman spectrum of natural hashemite, obtained for the first time, shows the position of the fundamental bands for the chromate anion vibrations. The bands related to the stretching vibrations (ν₁, ν₃) occur at 864 cm^-1 and in 871-909 cm^-1 regions, whereas the bending vibrations (ν₂, ν₄) are visible in the 346-360 cm^-1 and 400-422 cm^-1 range, respectively. Received results allowed to observe a gradual shift of bands in baryte-hashemite solid solution as a consequence of the substitution by different cations. The position of bands depends on the Cr/S ratio in analysed samples, and it is determined by differences in atomic mass, and ionic radii between Cr⁶⁺ and S⁶⁺, which affect changes in the strength and length of bonds. The occupancy of the same atomic position by two different cations enables to notice variations of polyhedra geometry, and unit cell parameters despite that baryte and hashemite are isostructural and crystallize in the same Pnma space group. We also confirm that the immobilization of the toxic (CrO₄)^2- ion in the baryte structure may occur directly without oxygen state reduction, we propose to using a baryte-hashemite solid solution as a reservoir for the incorporation of Cr as an environmental pollutant.

Synthesis and Characterization of the Homoleptic Octahydrotriborate Complex Cr(B3H8)2 and Its Lewis Base Adducts

Solvate-free sodium octahydrotriborate, NaB3H8, is prepared on a 20 gram scale from sodium amalgam and diborane in diethyl ether. This substance, which is chemically related to borohydride-based compounds being investigated as hydrogen storage materials, is also useful for the preparation of transition-metal complexes bearing B3H8 ligands. Treatment of CrCl3 with NaB3H8 affords a thermally unstable purple liquid thought to be a chromium(III) hydride of stoichiometry CrH(B3H8)2. This hydride converts rapidly at room temperature to the chromium(II) complex Cr(B3H8)2, which adopts a square-planar structure in which four hydrogen atoms form the coordination sphere of the chromium atom. This chromium(II) species forms six-coordinate Lewis base adducts Cr(B3H8)2L2, where L is Et2O, THF, or PMe3; the first two of these adopt trans geometries, whereas the latter is cis. Volatile Cr(B3H8)2 is the first homoleptic transition-metal complex of the octahydrotriborate anion, and it is an excellent single-source precursor for the chemical vapor deposition of thin films of CrB2 at temperatures as low as 200 degrees C. Crystal structures of the new complexes are reported.