Ulli Englert

In the absence of electronic reasons for distortion and with its ligands idealized as spherical, the hexaaquanickel(II) cation may be regarded as a simple example for octahedral metal coordination. In structurally characterized derivatives, the cation must adopt a lower symmetry in which it may well be distorted. We investigate hexaaquanickel(II) 3-carboxy-4-hydroxybenzenesulfonate dihydrate, a solid in which the cation occupies a center of inversion and subtends classical O–H···O hydrogen bonds with anions and co-crystallized water. Based on high-resolution X-ray diffraction, we analyze the resulting experimental electron density in order to understand why one of the three independent Ni–O coordinative bonds is significantly shorter than the others. The electron density and its derived properties in the bond critical points do not indicate correlation between hydrogen and coordinative bonds. The electrostatic potential derived from the experimental density reflects the electron distribution in the d orbitals and suggests that intra-cation repulsions and attractions are responsible for the distorsion of the cation. The quality of our experimental results is further underlined by a very good match between the measured electron density in the bond critical points of the Ni–O coordinative bonds and a previous report in the literature.

Organotellurium(II) compounds contain two σ‐holes that may be fine‐tuned through substituents. This work investigates the structural chemistry of two new chalcogen bond (ChB) aggregates of organotellurium(II) compounds and the strong bases 1,4‐diazabicyclo[2.2.2]octane (DABCO) and quinuclidine. Two different structural motifs were encountered: an oligomeric ChB aggregate between bis(pentafluorophenyl) telluride and the acceptor DABCO, in which each Te atom partakes in two short Te···N contacts, and a molecular adduct between the unsymmetrical (pentafluorophenyl)(phenyl) telluride and quinuclidine, in which only the more electrophilic σ‐hole forms the interaction. The latter ChB represents the shortest intermolecular Te···N contact characterized by diffraction methods. The calculated electron density of the compound was obtained using the experimentally established geometry, and the electrostatic potential of the ChB donor confirmed that electron‐withdrawing organic residues result in a more pronounced associated σ‐hole.

The formation of CuI-vinyl π-complexes relies on a cooperative interplay between ligand polarization and donor softness. To investigate this concept experimentally, a series of vinylic tris(thioether) ligands CH₂═CH-E(CH₂SPh)₃ (E = C, Si, Ge) was synthesized and characterized in solution by multinuclear NMR spectroscopy, with L1-Si additionally studied in the solid state by X-ray crystallography. While the purely carbon-based ligand does not form a CuI complex under comparable conditions, the silicon- and germanium-substituted derivatives yield the one-dimensional coordination polymers [Cu(μ₂-I)₂Cu(μ-L1-E)₂]_n, featuring Cu(μ₂-I)₂Cu rhomboids bridged by thioether donors and a side-on π-bound vinyl group occupying the fourth coordination site. High-resolution X-ray diffraction data for L1-Si and CP-Si enabled multipole refinement and topological analysis of the experimental electron density. This approach directly confirms the vinyl-Cu(I) π-interaction and reveals subtle, polarization-dependent changes in bond topology induced by main-group substitution. Comparative studies on the isostructural germanium polymer indicate weaker π-bonding but uncover an unprecedented, reversible temperature-dependent site exchange of the Ge atom accompanied by thioether inversion. Overall, the electron-density analysis establishes main-group element polarization as a decisive factor in enabling vinyl-Cu(I) π-bonding and provides a robust experimental framework for the rational design of multifunctional Cu(I) coordination materials.

A series of trigonal prismatic Co( ii ) complexes exhibit different uniaxial anisotropies and dynamic magnetic properties, which are affected by the bite angle β of O–Co–O and Co–O bonds.

Halogen bonds (XBs) represent strongly directional interactions, relevant for bond activation and often quoted in the context of Crystal Engineering. Bifurcated XBs, albeit rare, are unambiguously established in crystalline solids but do not correspond to an energetically favorable arrangement at the molecular level. Here the dynamic behavior of such a system, jointly described by experiment and theory, reconciles both aspects: Experiment shows that the cocrystal of 1,4‐diiodobenzene and 1,4‐dinitrobenzene with a symmetric bifurcated XB undergoes a rarely observed reversible crystal‐to‐crystal transition at 130 K. The resulting low‐ temperature phase features an asymmetric bifurcated XB, a first step toward a more linear arrangement. Theoretical calculations based on revised van der Waals density functional theory and diffraction experiments conducted at high resolution agree remarkably well in their interpretation of transition temperature and relevance of interaction types. The XB is fluxional and entropy‐favored in the symmetric high‐ temperature phase and locks into an asymmetric geometry with slightly more favorable Gibbs free energy at low temperature; the energy differences involved are extremely small. Theoretically derived and experimentally observed electron densities agree that the perceived directional interactions are weak rather than decisive synthons. Stacking of alternating diiodobenzene and dinitrobenzene molecules represents the most relevant contact.

Ortho ‐carboxyl substitution increases the acidity of hypervalent diaryliodonium salts and makes them useful catalysts in the synthesis of bis(indolyl)methanes under mild conditions, with yields up to 98%. The halogen bond‐assisted hydrogen bonding was demonstrated by crystal structure analysis. The interaction distance of 2.619 Å between I ^(III) and the carboxyl O atom indicates either a long covalent or a very short halogen bond; analysis of the electron density shows the latter description is appropriate.