High- and Low-Temperature Modifications of Sc3RuC4 and Sc3OsC4—Relativistic Effects, Structure, and Chemical Bonding

Trimorphic TaCrP – A diffraction and 31P solid state NMR spectroscopic study

The metal-rich phosphide TaCrP forms from the elements by step-wise solid state reaction in an alumina crucible (maximum annealing temperature 1180 K). TaCrP is trimorphic. The structural data of the hexagonal ZrNiAl high-temperature phase (space group P 6 ‾ 2 m $P\overline{6}2m$ ) was deduced from a Rietveld refinement. At room temperature TaCrP crystallizes with the TiNiSi type (Pnma, a = 623.86(5), b = 349.12(3), c = 736.78(6) pm, wR = 0.0419, 401 F 2 values, 20 variables) and shows a Peierls type transition below ca. 280 K to the monoclinic low-temperature modification (P121/c1, a = 630.09(3), b = 740.3(4), c = 928.94(4) pm, β = 132.589(5)°, wR = 0.0580, 1378 F 2 values, 57 variables). The latter phase transition is driven by pairwise Cr–Cr bond formation out of an equidistant chain in o-TaCrP. The phase transition was monitored via different analytical tools: differential scanning calorimetry, powder synchrotron X-ray diffraction, magnetic susceptibility measurements and 31P solid state NMR spectroscopy.

Scandium–copper–indides deriving from the ZrNiAl and MnCu2Al type structures

Phase analytical studies in the Sc–Cu–In system led to samples of the solid solutions ScCu1–x–y In1+x and ScCu2–x In which were studied by X-ray powder diffraction. At room temperature the compounds ScCu1–x–y In1+x crystallize with the ZrNiAl type, space group P 6 ‾ $\overline{6}$ 2m. Exemplarily, the structure of ScCu0.76In1.17 was refined from single crystal X-ray diffractometer data, revealing strong anisotropic displacements for the scandium atoms and a mixed occupied Cu/In site. Superstructure formation is observed at low temperatures. The ScCu0.78In1.14 and ScCu0.76In1.16 structures were refined from diffraction data recorded at 90 K. Both compounds adopt the HfRhSn type, space group P 6 ‾ $\overline{6}$ 2c, a klassengleiche subgroup of index 2; doubling of the subcell c axis. The Cu/In filled trigonal Sc6 prisms are strongly distorted in the superstructure, resulting from pairwise dislocation of the Cu/In atoms from ideal positions within an equidistant chain to shorter (311.0 pm) and longer (392.8 pm) Cu/In–Cu/In distances. Single crystal data of the Heusler phases ScCu1.95In and ScCu1.94In show small degrees of copper vacancies.

Molecular QTAIM Topology Is Sensitive to Relativistic Corrections

The topology of the molecular electron density of benzene dithiol gold cluster complex Au₄ -S-C₆ H₄ -S'-Au'₄ changed when relativistic corrections were made and the structure was close to a minimum of the Born-Oppenheimer energy surface. Specifically, new bond paths between hydrogen atoms on the benzene ring and gold atoms appeared, indicating that there is a favorable interaction between these atoms at the relativistic level. This is consistent with the observation that gold becomes a better electron acceptor when relativistic corrections are applied. In addition to relativistic effects, here, we establish the sensitivity of molecular topology to basis sets and convergence thresholds for geometry optimization.

Crystal structure resolution of an insulator due to the cooperative Jahn-Teller effect through Bader's theory: the challenging case of cobaltite oxide Y114

Cobaltite YBaCo4O7, abbreviated as Y114, is one of the most thoroughly investigated perovskites, owing largely to its interesting magnetic properties. Y114 is an insulator as a result of the cooperative Jahn-Teller effect, where one electron jumps quickly from one cobalt site to another, making it impossible to experimentally assign the correct oxidation state for each of the two cobalt sites. The present study solved the ambiguity by means of state-of-the-art DFT calculations. The two cobalt sites were differentiated through an analysis of charge density within the framework of the quantum theory of atoms in molecules.

The modulated structure of intermediate-valent CeCoGa

CeCoGa was synthesized by melting of the elements in an arc-melting furnace as well as in a sealed niobium tube in an induction furnace. A further annealing step improves the purity and crystallinity of the samples significantly. Its structure was refined on the basis of single-crystal X-ray diffractometer data at different temperatures. Already at room temperature CeCoGa crystallizes in a superstructure of the HT-CeCoAl type. This superstructure can be described in the (3+1)D superspace group C2/m(α0γ)00; α=2/3, γ=1/3 with a temperature independent q-vector (Z=4). For the 300 K data (also for 90 K) the commensurate modulated structure could be refined with 1336 F 2 values, 56 variables and residuals of wR=0.0348 for the main and wR=0.0605 for the satellites of 1st order [a=1101.7(1), b=436.0(1) and c=482.4(1) pm, β=103.2(1)°]. Furthermore a description in a transformed 3D supercell with the space group C2/m and Z=12 is possible. For the 90 K data this 3D supercell was refined with 1289 F 2 values, 56 variables and a residual of wR=0.0409 [a=1618.8(1), b=435.3(1) and c=1094.1(1) pm, β=119.3(1)°]. The relation of the HT-CeCoAl type structure, the (3+1)D modulated and the 3D supercell structure are discussed on the basis of a group-subgroup relation. By thermal analysis and magnetic measurements the phase transition temperature to the HT-CeCoAl type structure is stated slightly above 475 K. Furthermore intermediate cerium valence was identified by the magnetic susceptibility.

Topology of the electron density of d0 transition metal compounds at subatomic resolution

Accurate X-ray diffraction experiments allow for a reconstruction of the electron density distribution of solids and molecules in a crystal. The basis for the reconstruction of the electron density is in many cases a multipolar expansion of the X-ray scattering factors in terms of spherical harmonics, a so-called multipolar model. This commonly used ansatz splits the total electron density of each pseudoatom in the crystal into (i) a spherical core, (ii) a spherical valence, and (iii) a nonspherical valence contribution. Previous studies, for example, on diamond and α-silicon have already shown that this approximation is no longer valid when ultrahigh-resolution diffraction data is taken into account. We report here the results of an analysis of the calculated electron density distribution in the d(0) transition metal compounds [TMCH3](2+) (TM = Sc, Y, and La) at subatomic resolution. By a detailed molecular orbital analysis, it is demonstrated that due to the radial nodal structure of the 3d, 4d, and 5d orbitals involved in the TM-C bond formation a significant polarization of the electron density in the inner electronic shells of the TM atoms is observed. We further show that these polarizations have to be taken into account by an extended multipolar model in order to recover accurate electron density distributions from high-resolution structure factors calculated for the title compounds.

On the chemical shifts of agostic protons

Agostic hydrogen atoms in planar d(8) transition metal complexes display a remarkable wide range of chemical shifts from +5 to -10 ppm in the proton NMR spectra. It is therefore surprising that a simple recipe can be elaborated to predict the influence of the local electronic structure of the metal atom on the shielding of the coordinating protons: In cases where the agostic hydrogen atom is pointing to a local Lewis acidic center at the metal the (1)H NMR signal is shifted upfield relative to the scenario where the proton is opposing a local charge concentration at the metal. To trace the physical origin of this empirical relationship, a systematic study has been performed to understand how the (i) topology of the electron density and (ii) orientation of the magnetic field vector, B0, control the paratropic or diatropic characteristics of the induced current density at the metal atom and thus the shielding or deshielding of the agostic protons.