Christoph Hauf

On the Electronic Structure of NiII Complexes That Feature Chelating Bisguanidine Ligands

In this work we report on the syntheses and properties of several new Ni complexes featuring the chelating bisguanidines bis(tetramethylguanidino)benzene (btmgb), bis(tetramethylguanidino)naphthalene (btmgn), and bis(tetramethylguanidino)biphenyl (btmgbp) as ligands. All complexes were structurally characterized by single-crystal X-ray diffraction and quantum chemical calculations. A detailed inspection of the magnetic susceptibility of [(btmgb)NiX(2)] and [(btmgbp)NiX(2)] (X=Cl, Br) revealed a linear temperature dependence of chi(-1)(T) above 50 K, which was in agreement with a Curie-Weiss-type behavior and a triplet ground state. Below approximately 25 K, however, magnetic susceptibility studies of the paramagnetic d(8) Ni complexes revealed the presence of a significant zero-field splitting (ZFS) that results from spin-orbit mixing of excited states into the triplet ground state. The electronic consequences that might arise from the mixing of states as well as from a possible non-innocent behavior of the ligand have been explored by an experimental charge density study of [(btmgb)NiCl(2)] at low temperatures (7 K). Here, the presence of ZFS was identified as one potential reason for the flat angle-spherical Cl-Ni-Cl deformation potential and the distinct differences between the angle-spherical X-Ni-X valence angles observed by experiment and predicted by DFT. An analysis of the topology of the experimentally and theoretically derived electron-density distributions of [(btmgb)NiCl(2)] confirmed the strong donor character of the bisguanidine ligand but clearly ruled out any significant non-innocent ligand (NIL) behavior. Hence, [(btmgb)NiCl(2)] provides an experimental reference system to study the mixing of certain excited states into the ground state unbiased from any competing NIL behavior.

A Unifying Bonding Concept for Metal Hydrosilane Complexes

Experimental and theoretical charge density studies and molecular orbital analyses suggest that the complexes [Cp2Ti(PMe3)SiH2Ph2] (1) and [Cp2Ti(PMe3)SiHCl3] (2) display virtually the same electronic structures. No evidence for a significant interligand hypervalent interaction could be identified for 2. A bonding concept for transition-metal hydrosilane complexes aims to identify the true key parameters for a selective activation of the individual M-Si and Si-H bonds.

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.

Probing the Zintl-Klemm concept: a combined experimental and theoretical charge density study of the Zintl phase CaSi.

The nature of the chemical bonds in CaSi, a textbook example of a Zintl phase, was investigated for the first time by means of a combined experimental and theoretical charge density analysis to test the validity of the Zintl-Klemm concept. The presence of covalent Si-Si interactions, which were shown by QTAIM analysis, supports this fundamental bonding concept. However, the use of an experimental charge density study and theoretical band structure analyses give clear evidence that the cation-anion interaction cannot be described as purely ionic, but also has partially covalent character. Integrated QTAIM atomic charges of the atoms contradict the original Zintl-Klemm concept and deliver a possible explanation for the unexpected metallic behavior of CaSi.

Possible indicators for low dimensional superconductivity in the quasi-1D carbide Sc3CoC4

The transition metal carbide Sc3CoC4 consists of a quasi-one-dimensional (1D) structure with [CoC4]∞ polyanionic chains embedded in a scandium matrix. At ambient temperatures Sc3CoC4 displays metallic behavior. At lower temperatures, however, charge density wave formation has been observed around 143 K which is followed by a structural phase transition at 72 K. Below Tconset = 4.5 K the polycrystalline sample becomes superconductive. From Hc1(0) and Hc2(0) values we could estimate the London penetration depth (λL ≊ 9750 A) and the Ginsburg-Landau (GL) coherence length (ξGL ≊ 187 A). The resulting GL-parameter (κ ≊ 52) classifies Sc3CoC4 as a type II superconductor. Here we compare the puzzling superconducting features of Sc3CoC4, such as the unusual temperature dependence i) of the specific heat anomaly and ii) of the upper critical field Hc2(T) at Tc, and iii) the magnetic hysteresis curve, with various related low dimensional superconductors: e.g., the quasi-1D superconductor (SN)x or the 2D transition-metal dichalcogenides. Our results identify Sc3CoC4 as a new candidate for a quasi-1D superconductor.

On the Interplay Between Real and Reciprocal Space Properties

The relationship between charge density distributions and physical properties in solids is highly complex and usually not obvious. It is therefore the aim of this Chapter to outline concepts how to explore and to analyze the interplay of real space properties (e.g. the charge density distribution or its Laplacian) and reciprocal space properties in solids (e.g. electronic conductivity, superconductivity) by means of charge density analyses. In our case study, we will focus on quasi-one dimensional organometallic carbides, which are textbook examples of extended systems displaying pronounced orbital interactions and anisotropic physical properties in real and reciprocal space. We therefore investigated the electronic structures of the complex carbides Sc3 TC4 (T=Fe (1), Co (2), Ni (3)) by combined theoretical and experimental charge density studies. The structures of these organometallic carbides are closely related and display one-dimensional infinite TC4 ribbons embedded in a scandium matrix. Our study highlights that despite the structural similarities of 1–3 even tiny differences in the electronic band structure are faithfully recovered in the properties of the Laplacian of the electron density. In our case, the shift of the Fermi level to higher energies for the Co(d 9) and Ni(d 10) carbides 2 and 3 relative to the Fe(d 8) analogue 1 is reflected in the charge density picture by a significant change in the polarization pattern displayed by the valence shell charge concentrations (VSCC) of the individual 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)/Z(Ni)=26/27/28). We further demonstrate that the presence of an axial VSCC at the iron atom is due to localized d z 2 states near the Fermi energy and reflected by a high electronic heat capacity at low temperatures (Sommerfeld coefficient γ=17mJ/K2mol in 1). On contrast, the lack of a narrow conduction band (and axial VSCCs at the transition metal) could be correlated in 2 and 3 with their smaller Sommerfeld coefficients (γ=5.7 and 7.7mJ/K2mol, respectively). Finally, we demonstrate that also the cobalt carbide 2 can be discriminated from its isotypic nickel congener 3 on the basis of its electronic properties. Indeed, only 2 is superconducting below 4.5K and displays a structural phase transition around 70K. Hence, this Chapter should help filling the gap between the various chemical and physical viewpoints on the interplay of chemical bonding and physical properties in solids.