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.

In general, C À H bonds can be considered chemically inert as a result of their strength, nonpolar nature, and low polarizability. Since the pioneering work of La Placa and Ibers in 1965, who reported the close approach of a CÀH bond to a transition-metal center, there have been many attempts to trace the microscopic control parameters of such C À H activation processes by metal atoms in general.[1] In particular, complexes containing side-on-coordinated (h 2 -CH) moieties next to a transition metal are the focus of intensive research as they allow the systematic study of the CÀH activation phenomenon in molecules and solids in their electronic ground states. Furthermore, M···H À C interactions (M = transition metal) play a key role in the performance of several industrially relevant catalytic processes, such as olefin polymerization.[2]In the course of a systematic analysis of such M···HÀC interactions, Brookhart and Green coined the expression agostic interactions to "discuss the various manifestations of covalent interactions between C À H groups and transitionmetal centers in organometallic compounds". [3a,b] In case of d 0 early-transition-metal alkyl or amido complexes, the strength of agostic interactions is mainly controlled by 1) the local Lewis acidity of the metal center, 2) the extent of negative hyperconjugative delocalization of the M À C/M À N bonding electrons, and 3) to a smaller degree by s(M ! H À C) donation.[3c, 4] For agostic late-transition-metal complexes, however, the control parameters are less clear. We therefore synthesized a variety of new Spencer-type [5] nickel alkyl cations 2 b-d by protonation of the corresponding olefin complexes 1 b-d to study the nature of their pronounced agostic interactions by combined experimental and theoretical charge density studies (Scheme 1).Re-examination of the classic Spencer-type complex [EtNi(dtbpe)]showed a fast rotation of the b-agostic methyl moiety in solution[6] and a systematic crystallographic disorder in the solid state, thus preventing a detailed investigation of the bonding properties of this agostic textbook example by experimental charge-density studies. We therefore replaced the ethylene moiety in 1 a by the sterically more demanding norbornyl (nbe) and dicyclopentadienyl (DCp) ligands. Protonation of 1 b-d yielded the agostic complexes 2 b-d, which all have a significantly reduced fluxional behavior in solution. Furthermore, single crystals of excellent quality could be obtained for 2 b-d, which even allowed an experimental charge-density analysis of 2 c.

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.

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.

In general, it is assumed that the reaction between silanes and late transition metal fragments yields silyl hydride species as oxidative addition products. However, the silane complex Ni(iPr 2 Im) 2 (SiHMePh 2 ) (iPr 2 Im = 1,3-diisopropylimidazolin-2-ylidene) (3a), might represent one of the rare systems where a stable η 2 -(Si-H)Ni intermediate of the oxidative addition process has been isolated. Indeed, 3a is characterized by an acute ЄSi-Ni-H angle of * Prof. Dr. U. Radius,

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.

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.

Within the series of ternary rare-earth transition metal carbides Sc 3 TC 4 (T = Fe, Co, Ni) only the Co congener displays a structural phase transition at 72 K and an onset of bulk superconductivity at 4.5 K. In this paper we present the results of a detailed analysis of the structural, electronic, and vibrational properties of the low-temperature phase of Sc 3 CoC 4 that represents one of the few well-documented examples of a quasi one-dimensional (1D) superconductor. Variable temperature neutron powder diffraction and low temperature X-ray diffraction experiments were performed in order to confirm the subtle structural distortions during the phase transition. The results of * Dr. G. Eickerling