A norganisch-C hem isches Institut, U niversität M ünster, W ilhelm -K lem m -Straße 8, D -48149 M ünster, G erm any Z. N aturforsch. 52 b, 1023 -1 0 3 0 (1997); received M ay 6, 1997 C rystal Structure, T ransition M etal A tom s Forty eight new com pounds RT2Z n 2o w ere prepared by annealing cold-pressed pellets o f the elem ental com ponents in an argon atm osphere. They crystallize with the cubic CeCr^A bo type structure (Fd3m , Z = 8), w hich was refined from single-crystal diffractom eter data o f TbFeiZn^o (a = 1411.1(1) pm ), Y R u2Z n 2o (a = 1422.6(1) pm ), D yR u2Z n 2o (a = 1422.1(1) pm ), G dC o2Z n 2o (a = 1406.0(1) pm ), D y R h2Z n 2o (a = 1418.2(1) pm ), a n d T m N i2Z n 2o (a= 1401.6(1) pm ) to conventional residuals varying betw een R = 0.011 and R -0.024. The com pounds have a tendency for tw inning, thus m im icking hexagonal sym m etry, w ith the cubic [111] axis as the axis w ith the pseudohexagonal sym m etry. M inor inconsistencies in the cell volum es o f these com pounds indicate slight deviations from the ideal com position. N evertheless, the five atom ic sites o f this structure w ere found to be fully occupied w ithin the error lim its w ith the exception o f one zinc site o f T m N i2Z n2o-The coordination for the site o f the rare earth atom s is a Frank-K asper polyhedron w ith coordination num ber (CN ) 16. The transition metal atom s occupy a site w ith icosahedral zinc coordination (CN 12). Two o f the three zinc sites are in pentagonal prism atic coordination o f zinc atom s, capped by rare earth and/or transition metal atom s (C N 12), w hile the third zinc site has 12 zinc neighbors form ing a hexagonal prism , w hich is capped by tw o rare earth atom s (C N 14).
The nature of chemical bonding in the complex carbides Sc3[Fe(C2)2] (1) and Sc3[Co(C2)2] (2) has been explored by combined experimental and theoretical charge density studies. The structures of these organometallic carbides contain one-dimensional infinite TC4 (T = Fe, Co) ribbons embedded in a scandium matrix. Bonding in 1 and 2 was studied experimentally by multipolar refinements based on high-resolution X-ray data and compared to scalar-relativistic electronic structure calculations using the augmented spherical wave method. Besides substantial covalent T-C bonding within the TC4 ribbons, one also observes discrete Sc-C bonds of noticeable covalent character. Furthermore, our study highlights that even tiny differences in the electronic band structure of solids might be faithfully recovered in the properties of the Laplacian of the experimental electron density. In our case, the increase of the Fermi level in the organometallic Co(d9) carbide 2 relative to its isotypic Fe(d8) species 1 is reflected in the charge density picture by a significant change in the polarization pattern displayed by valence shell charge concentrations of the transition metal centers in the TC4 units. Hence, precise high-resolution X-ray diffraction data provide a reliable tool to discriminate and analyze the local electronic structures of isotypic solids, even in the presence of a severe coloring problem (Z(Fe)/Z(Co) = 26/27).
The reaction of the benzannulated bisstannylene ligand 2 with Sn O or Pb O generated in situ gave the pincer complexes 3 and 4. Both complexes have been characterized by X-ray diffraction and multinuclear NMR spectroscopy. A divalent state has been found by Mössbauer spectroscopy for the tin atoms in complexes 3 and 4.
The electronic structures of the isotypic carbides Sc3TC4 (see picture; T=Fe, Co, Ni) are investigated by theoretical and experimental charge‐density studies. Even tiny differences in the electronic band structure of these solids are reflected in the properties of the Laplacian of the experimental electron density. Only the cobalt carbide is superconducting below 4.5 K and displays a structural phase transition around 70 K.
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