The nature of agostic interactions in complexes of lithium and early transition metals

Die in der metallorganischen Komplexchemie als agostisch bekannten Wechselwirkungen zwischen einem Zentralmetall und der C-H-Bindung eines Liganden besitzen große Relevanz für das Verständnis wichtiger Prozesse, wie z. B. der C-H-Aktivierung sowie der Ziegler-Natta-Katalyse, sind aber dennoch bis heute theoretisch nur unzureichend verstanden. Im Rahmen der Dissertation wurden daher die experimentellen und theoretischen Elektronendichten einer Reihe von agostischen Alkylkomplexen des Lithiums und der frühen Übergangsmetalle bestimmt und mit Hilfe des "Atoms in Molecules"-Ansatzes von Bader bezüglich ihrer Topologie analysiert. Dabei ergab sich, daß in beiden Verbindungsklassen nicht direkte M...H-C-Wechselwirkungen die entscheidende Rolle spielen und zu den beobachteten, strukturellen Verzerrungen führen, sondern stattdessen als eigentliche Triebkraft die Delokalisierung von Elektronen der M-C-Bindung über die gesamte Alkyleinheit fungiert. Im Falle der frühen Übergangsmetalle konnte zudem gezeigt werden, daß dort lokale Ladungskonzentrationen am Zentralmetall eine Schlüsselfunktion einnehmen und diese durch die geeignete Wahl von Liganden manipuliert werden können.So-called agostic interactions between (transition) metal centers and C-H groups of appended ligands are ubiquitous in organometal chemistry and have central relevance to important processes like C-H activation and Ziegler-Natta catalysis. To explore the true nature of this phenomenon experimental and theoretical electron densities of agostic alkyl complexes of lithium and early transition metals were determined and analyzed topologically, using the "Atoms in Molecules" method of Bader. The studies revealed that the observed geometrical distortions in these compounds primarily arise from delocalization of the M-C bonding electrons over the entire alkyl fragment, with direct M...H-C interactions playing only a minor role in stabilizing the structure. In addition, ligand-induced local charge concentrations on transition metal centers were shown to be crucial in the development of agostic interactions

Comparison of Hydrogen Elimination from Molecular Zinc and Magnesium Hydride Clusters

In analogy to the previously reported tetranuclear magnesium hydride cluster with a bridged dianionic bis-beta-diketiminate ligand, a related zinc hydride cluster has been prepared. The crystal structures of these magnesium and zinc hydride complexes are similar: the metal atoms are situated at the corners of a tetrahedron in which the vertices are bridged either by dianionic bis-b-diketiminate ligands or hydride ions. Both structures are retained in solution and show examples of H-center dot center dot center dot H- NMR coupling (Mg: 8.5 Hz; Zn: 16.0 Hz). The zinc hydride cluster [NN-(ZnH)(2)](2) thermally decomposes at 90 degrees C and releases 1.8 equivalents of H-2. In contrast to magnesium hydride clusters, there is no apparent relationship between cluster size and thermal decomposition temperature for the zinc hydrides. DFT calculations reproduced the structure of the zinc hydride cluster reasonably well and charge density analysis showed no bond paths between the hydride ions. This contrasts with calculations on the analogous magnesium hydride cluster in which a counter-intuitive H-center dot center dot center dot H- bond path was observed. Forcing a reduced H-center dot center dot center dot H- distance in the zinc hydride cluster, however, gave rise to a H-center dot center dot center dot H- bond path. Such weak interactions could play a role in H-2 desorption. The presumed molecular product after H-2 release, a Zn(I) cluster, could not be characterized experimentally but DFT calculations predicted a cluster with two localized Zn-Zn bonds

Elucidation of the bonding in Mn(η²-SiH) complexes by charge density analysis and T₁ NMR measurements: asymmetric oxidative addition and anomeric effects at silicon

The bonding in Mn(η2-SiH) complexes is interpreted in terms of an asymmetric oxidative addition whose extent is controlled by the substitution pattern at the hypercoordinate silicon centre, and especially by the ligand trans to the η2-coordinating SiH moiety

DIBAH‐Mediated Amide/Hydride Transformation in ansa‐Lanthanidocene(III) Complexes

DIBAH-mediated amide/hydride transformation in ansa-lanthanoidocene(III) complexes / W. Scherer ... – In: Angewandte Chemie / Internat. ed. 42. 2003. S. 574-57