Chemical bonding analysis and properties of La 7Os 4C 9—A new structure type containing C- and C 2-units as Os-coordinating ligands

Abstract

The new ternary carbide La 7Os 4C 9 was prepared by argon arc-melting of the elements followed by subsequent heat treatment at 900 °C for 250 h. The compound crystallizes monoclinic, in the space group C2/ m ( a=1198.5(2) pm, b=542.0(1) pm, c=1196.2(2) pm, β=111.04(1)°, V=725.2(2)×10 6 pm 3, Z=2). The structure was determined from single crystal X-ray diffraction data and refined to a residual of R 1=0.02 (w R 2=0.03) for 4812 unique reflections and 64 variable parameters. Electrical resistivity and magnetic susceptibility measurements characterize the compound as a Pauli-paramagnetic metal. The crystal structure contains bridging C- and terminal C 2-units as Os-coordinating ligands, thereby forming polyanions ∞ 1 [ Os 4 ( C 2 ) 2 C 5 ] running along the [101] direction. The polyanions are composed of alternating Os(C 2)C 2 and OsC 3 units with the transition metal in distorted trigonal planar coordination. Charge compensation is ensured by La cations which are situated in-between the polyanions. The carbon–carbon bond (131 pm) within the C 2 pairs is slightly shorter than the value of a common C–C double bond, and is discussed on the basis of COHP curves on the one side, and with ELI-D and electron density distributions on the other side. The method of partial ELI-D decomposition is shown to be well suited for the characterization of separated DOS structures in terms of chemical bonding signatures provided by ELI-D. The Os–La interactions are shown to be of a polar multicenter-bonding type with Os playing the role of the electron donor. Compared to an acetylide the C 2 species were found to possess a significantly reduced bond order and an enhanced number of electrons in lone pair type spatial regions. This type of species cannot be simply classified in terms of model pictures such as C 2 2− and C 2 4−, respectively.