Li2B12C2 und LiB13C2: farblose borreiche Boridcarbide des Lithiums

Synthesis and crystal structures of the new ternary borides Fe3Al2B2 and Ru9Al3B8 and the confirmation of Ru4Al3B2 and Ru9Al5B8−x (x≈2)

Single crystals of the new ternary borides Fe3Al2B2 and Ru9Al3B8 were obtained from the elements at 1900°C. Both compounds represent new structure types which combine well-known features of binary and ternary borides of transition metals in combination with aluminum. The crystal structure of Fe3Al2B2 (P2/m, Z=2, a=5.724, b=2.857, c=8.723 Å, β=98.57°) contains tetramers of face-sharing trigonal prisms BFe6 with a B4 unit in trans-configuration. The tetrameric units are separated by Al-atoms which occupy all remaining rectangular sites of the trigonal prisms. The structure can be derived from Fe2AlB2 by insertion of additional FeAl fragments in a bcc arrangement. The crystal structure of Ru9Al3B8 (P6̅2m, Z=1, a=9.078, c=2.913 Å) combines zig-zag chains of boron atoms made of face-sharing trigonal prisms BFe6 and isolated BFe6 units. Three of these chains are connected by common corners to rods running in direction [001]. The rods are linked to a three-dimensional framework by isolated prisms via common edges. Again, Al occupies the capping positions of the trigonal prisms. Ru9Al3B8 is the second representative for the combination of boron zig-zag chains and isolated B atoms. The existence of Ru4Al3B2 (P4/mmm, Z=2, a=8.515, c=2.924 Å) and Ru9Al5B8−x (P4/m, Z=1, a=8.741, c=2.923 Å) were confirmed and the crystal structures refined. High quality data reveal a stoichiometric composition for Ru4Al3B2, while in Ru9Al5B8−x there is a significant underoccupation (i.e. x≈2) of the central boron site within the B4 units. The crystal structures of all four compounds represent examples for the combination of CsCl and AlB2 fragments as they were frequently found for ternary borides of transition metals.

Syntheses of Mono- and Dinuclear Diiodoboryl Complexes of Platinum

Treatment of [Pt(PCy(3))(2)] (Cy = cyclohexyl) with BI(3) afforded trans-[(Cy(3)P)(2)Pt(I)(BI(2))] by the oxidative addition of a B-I bond. The title compound represents the first diiodoboryl complex and was fully characterized by NMR spectroscopy and X-ray diffraction analysis. The latter revealed a very short Pt-B distance, thus indicating a pronounced pi contribution to this bond. By the addition of another 1 equiv of BI(3) to trans-[(Cy(3)P)(2)Pt(I)(BI(2))], a new Pt species [(Cy(3)P)(I(2)B)Pt(mu-I)](2) was formed with concomitant buildup of the phosphine borane adduct [Cy(3)P-BI(3)]. The former is obviously obtained by abstraction of PCy(3) from trans-[(Cy(3)P)(2)Pt(I)(BI(2))] and the subsequent dimerization of two remaining fragments. Interestingly, the dimerization is reversible, and the dinuclear compound can be converted to trans-[(Cy(3)P)(2)Pt(I)(BI(2))] upon the addition of PCy(3).

Synthesis, crystal structure, and properties of two modifications of MgB(12)C(2)

Single crystals of two modifications of the new magnesium boride carbide MgB(12)C(2) were synthesized from the elements in a metallic melt by using tantalum ampoules. Crystals were characterized by single-crystal X-ray diffraction and electron microprobe analysis (energy-dispersive (EDX) and wavelength-dispersive (WDX) X-ray spectroscopy). Orthorhombic MgB(12)C(2) is formed in a Cu/Mg melt at 1873 K. The crystal structure of o-MgB(12)C(2) (Imma, Z=4, a=5.6133(10), b=9.828(2), c=7.9329(15) A, 574 reflections, 42 variables, R(1)(F)=0.0208, wR(2)(I)=0.0540) consists of a hexagonal primitive array of B(12) icosahedra with Mg atoms and C(2) units in trigonal-prismatic voids. Each icosahedron has six exohedral B--B and six B--C bonds. Carbon is tetrahedrally coordinated by three boron atoms and one carbon atom with a remarkably long C--C distance of 1.727 A. Monoclinic MgB(12)C(2) is formed in an Al/Mg melt at 1573 K. The structure of m-MgB(12)C(2) (C2/c, Z=4, a=7.2736(11), b=8.7768(13), c=7.2817(11) A, beta=105.33(3) degrees , 1585 reflections, 71 variables, R(1)(F)=0.0228, wR(2)(I)=0.0610) may be described as a distorted cubic close arrangement of B(12) icosahedra. Tetrahedral voids are filled by C atoms and octahedral voids are occupied by Mg atoms. The icosahedra are interconnected by four exohedral B--B bonds to linear chains and by eight interstitial C atoms to form a three-dimensional covalent network. Both compounds fulfill the electron-counting rules of Wade and Longuet-Higgins.