This contribution provides the first detailed analysis of the nature of the M–C σ-bond of three alkylated, isostructural group 4 (M = Ti, Zr, Hf) metallocenes, thereby elucidating individual peculiarities of each metal center in the catalytic conversion of olefins. Therefore, the subtle electronic differences of the individual M–C σ-bonds, which are considered crucial for several subprocesses in the coordinative polymerization of olefins, were examined by detailed experimental charge density studies. These studies provided measures of the increasing ionic character of the M–C bonds along the group 4 elements (Ti–C < Zr–C < Hf–C). These results are further supported by high-pressure diffraction studies showing that the predominantly ionic Hf–C bond is more compressible than the more covalent Zr–C bond in line with a smaller degree of electron localization in the valence shell of the hafnium relative to the zirconium atom along the M–C bond directions. The Ti–C bond displays the largest degree of electron localization in these group 4 metallocenes as witnessed by a pronounced bonded charge concentration in the valence shell of the titanium atom–a rare phenomenon in transition metal alkyls. All findings were then complemented by experimental and theoretical studies of the kinetic aspects of M–C σ-bond cleavage in group 4 metallocenes. These studies show that the entropy of activation is distinctly more negative for a Zr–C relative to a Hf–C bond dissociation. The combined results of the kinetic and electronic analysis herein shed new light on the different catalytic behavior of group 4 metallocenes with regard to the applied transition metal atom. In this context, deviations between zirconium- and hafnium-based catalysts concerning the catalytic activity and the stereoregularities became clearly explainable, just as the well-known “hafnium-effect” in the production of extraordinarily high molecular weight polypropylenes.

The layered material ZrSiTe is currently extensively investigated as a nodal-line semimetal with Dirac-like band crossings protected by nonsymmorphic symmetry close to the Fermi energy. A recent infrared spectroscopy study on ZrSiTe under external pressure found anomalies in the optical response, providing hints for pressure-induced phase transitions at ≈4.1 and ≈6.5 GPa. By pressuredependent Raman spectroscopy and x-ray diffraction measurements combined with electronic band structure calculations we find indications for two pressure-induced Lifshitz transitions with major changes in the Fermi surface topology in the absence of lattice symmetry changes. These electronic phase transitions can be attributed to the enhanced interlayer interaction induced by external pressure. Our findings demonstrate the crucial role of the interlayer distance for the electronic properties of layered van der Waals topological materials.Topological materials such as topological insulators [1], Dirac [2], Weyl [3][4][5] or line-node semimetals [6,7] are of great fundamental interest due to their exotic nature of electronic phases, and thus heavily investigated nowadays. They usually exhibit extraordinary material properties, for example, high carrier mobility and unusual magnetoresistance [8][9][10]. Topological non-trivial phases often occur in layered materials with weak interlayer bonding, where the single layers behave rather as isolated two-dimensional (2D) objects, enabling the exfoliation to atomically thin 2D crystals with numerous possible applications [11][12][13][14][15]. Since the forces between the layers of such structures are usually weak, they are highly compressible perpendicular to the layers, and a dimensional crossover from 2D to 3D can be induced by external pressure. Generally, layered materials are prone to pressure-induced phenomena, and electronic topological transitions [16] are expected to be induced [17][18][19] and were suggested to occur in layered BiTeBr [20], BiTeI [21, 22], 1T-TiTe 2 [23], and the group V selenides and tellurides Bi 2 Se 3 , Bi 2 Te 3 , Sb 2 Te 3 [24-27]. "Electronic transitions" in metals were first introduced by Lifshitz in 1960 as transitions where the topology of the Fermi surface (FS) changes as a result of the continuous deformation under high external pressure [16]. Examples for pressure-induced alterations of the FS topology are the conversion of an open Fermi surface, such as * These authors contributed equally.a corrugated cylinder-type Fermi surface typical for layered materials, to a closed one, or the appearance of a new split-off region of the FS. Importantly, the changes in the Fermi surface topology during such a so-called Lifshitz transition are not related to a change in the lattice symmetry [16].In this work we find indications for two Lifshitz transitions in the layered van der Waals material ZrSiTe under external pressure, resulting from the enhanced interlayer interaction. ZrSiTe belongs to the family of compounds ZrXY (X=Si, Ge, Sn and Y =O, S, Se, Te), which are...

We investigate pressure-induced structural changes to the Peierls-type distorted low-temperature phase of the low-dimensional Sc 3 CoC 4 as a possible origin of its pressure-enhanced superconductivity. By means of cryogenic high-pressure x-ray diffraction experiments we could reveal subtle, but significant structural differences between the low-temperature phase at ambient and elevated pressures. We could thus establish the structure of the superconducting phase of the title compound, which interestingly still shows the main features of the Peierls-type distorted low-temperature phase. This indicates that in contrast to other low-dimensional materials a suppression of periodic structural distortions is no prerequisite for superconductivity in the transition-metal carbide.

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density-functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS.

The nature of the interaction between chloromethanes CH 4-n Cl n and Pt(II) complexes has been studied by highpressure X-ray diffraction and infrared spectroscopy in combination with DFT calculations. In case of electron rich complexes such as d 8 -Pt(btz-N,N′)(phenyl)L with L = phenyl, Cl, Br and btz = 2,2′-Bi-5,6-dihydro-4H-1,3-thiazine stable chloroform adducts with bridging hydrogen atoms in the η 1 (C-H)Pt moieties were isolated which display highly activated C-H bonds. This activa-The activation of carbon-hydrogen bonds is usually hampered by their rather apolar covalent character and large bond dissociation energies. For example the C-H bond dissociation enthalpies in simple alkanes such as methane [DH 298 = 439.28(13) kJ mol -1 ] are virtually as large as in the H 2 molecule [DH 298 = 435.998(13) kJ mol -1 ] displaying the prototype of a strong covalent bond. [1] As a consequence, alkanes are neither good electron donors nor good acceptors since the σ(C-H) bonding orbital is low in energy while the antibonding σ*(C-H) orbital is high lying. Hence, C-H bonds are generally considered to be chemically rather inert and their selective activation remains a challenge in organometallic chemistry. [2][3][4] This obstacle can be overcome by metal-assisted C-H bond activation in cases where an alkane ligand coordinates either end-on (η 1 ) or side-on (η 2 ) to a metal-ligand fragment ML n (Scheme 1). [5,6] In case of electron-rich late transition metal complexes two bonding scenarios with short M···H br -C contacts are usually observed for methane and halomethane d 8 -Pt complexes, where H br denotes a bridging hydrogen atom. These are illustrated in case of the theoretical model systems (CH 3 ) 2 Pt(NH 3 )(CH 4 ) 1a and (CH 3 ) 2 Pt(NH 3 ) 2 ·(CHCl 3 ) 1b in Scheme 1. We note, that all DFT calculations were performed with ADF using the BP86 functional, the ZORA for the descrip- [a]