Neue Strukturverfeinerung und Eigenschaften von Cr3C2/Structure Refinement and Properties of Cr3C2

Cr3C2 was obtained from arc-melting of pellets made of carbon and chromium. The structure of Cr3C2 was determined by single crystal X-ray diffraction (Pnma, Z = 4, a = 553.99(6), b = 283.27(4), c = 1149.4(1) pm, R1 = 0.019 and wR2 = 0.037 for all collected reflections). The crystal structure contains isolated carbon atoms which reside inside of trigonal prismatic voids of metal atoms. The compound exhibits temperature independent paramagnetism. The electronic structure of Cr3C2 has been investigated using extended Hückel calculations.

New Magnesium Compounds RE2Cu2Mg (RE = Y, La -Nd, Sm, Gd -Tm, Lu) with Mo2FeB2 Type Structure

The title compounds were synthesized by reaction of the elements in sealed tantalum tubes in a water-cooled sample chamber in a high-frequency furnace. These magnesium intermetallics crystallize with the tetragonal Mo2FeB2 type structure, space group P4/mbm. The lattice pa­ rameters of all compounds were refined from X-ray powder data. Single crystal X-ray data yielded a = 792.09(6), c = 396.31(8) pm, wR2 = 0.0396, 315 F2 values for La2Cu2Mg, a = 778.30(5), c = 384.04(5) pm, wR2 = 0.0954, 214 F2 values for Nd2Cu2Mg, and a = 762.65(5), c = 374.09(3) pm, wR2 = 0.0566, 186 F2 values for Y2Cu2Mg with 12 variable parameters for each refinement. The RE2Cu2Mg structures can be described as an intergrowth of distorted A1B2 and CsCl related slabs of compositions RECu2 and REMg. Chemical bonding in La2Cu2Mg was investigated on the basis of extended Hückel calculations and compared to isotypic La2Cu2ln. This structure was also refined from single crystal X-ray data: P4/mbm, a = 780.8(2), c = 400.1(2) pm, wR2 = 0.0351, 211 F2 values and 12 variable parameters.

The Crystal Structure of WC Type ZrTe. Advantages in Chemical Bonding as Contrasted to NiAs Type ZrTe

Single crystals of WC type ZrTe were prepared from the elements. A single crystal structure determination of this structure type was performed for the first time: ZrTe (WC) crystallizes in the hexagonal space group P6̄m2 (No. 187), hP2, Z - 1, a - 377.06(5), c = 386.05(8) pm; 84 reflections, 5 variables, R(F) = 0.037. The distinctions in bonding for ZrTe (WC) and a hypothetical stoichiometric ZrTe crystallizing in the NiAs type structure were analyzed on the basis of extended Hückel calculations. Heteronuclear interactions contribute most strongly to the stability of both structures. Attractive Zr-Zr interactions energetically favour ZrTe (WC) relative to ZrTe (NiAs). The Fermi level of ZrTe (WC) resides in a local minimum of the DOS, whereas that of ZrTe (NiAs) intersects a local DOS maximum, and is pushed up by about 0.5 eV, expressing the decisive destabilization of NiAs type ZrTe. As a consequence, metal deficiency is observed for ZrTe (NiAs), in contrast to ZrTe (WC).

THEORETICAL STUDY OF CO ADSORPTION ON Ni(111), Pt(111) AND Pt/Ni(111) SURFACES

CO adsorption on a pseudomorphic Pt overlayer supported by Ni (111) has been studied with the use of extended Huckel calculations. Experimental information on the pure Pt (111) and Ni (111) single crystals was employed to select a consistent parameter set for our bimetallic system. This gives a good description of the chemisorption bond changes between the various systems considered in our study. The CO chemisorption energy on Pt/Ni (111) was found to be lowered in comparison with Pt (111) and Ni (111), in good agreement with experimental data on Pt-rich Pt–Ni surface alloys. This observation could be justified by the electronic changes of the Pt states (valence band broadening and decreasing density at the Fermi level). Indeed, they induce, in comparison with the pure substrates, a repulsion between Pt and CO although the 2π* population of the chemisorbed molecule increases. This points to the necessity of going beyond arguments based on an analysis of the 5σ donation and 2π* backdonation for a complete description of the chemisorption bond.

Analysis of 1H NMR Data for Arene-Metal Complexes Using Extended Huckel Calculations

This work reports the use of extended Hückel molecular orbital (EHMO) calculations to correlate pz electronic densities of aromatic carbons in group VI metal-bis(η6-arene) complexes with the respective 1H NMR chemical shifts. The effect of delocalization on the acceptor properties and stabilization of ligand orbitals is analyzed comparing complexes of naphthalene, biphenyle and fluorene.