Table of Contents:
The present work aimed to characterize the chemical bonding of transition metal complexes in uncommon bonding situations. An attempt was made to describe how attractive interactions between the fragments of different systems are achieved, and how these interactions can be understood. For this purpose different quantum-chemical methods were used: the number of (cluster) valence electrons (CVE or VE) could be ascertained by means of “classic” electron counting and the Natural Bond Orbital method (NBO) was employed to obtain preliminary indicators about partial charges and Wiberg bonding indices (WBIs). Molecular graphs were created and bond critical points (BCPs) were characterized by application of the Quantum theory of Atoms in Molecules (QTAIM). Thus based on the topological analysis of the electron density, which itself is based on physical principles, qualitative and quantitative predictions of the attractive interactions between atoms could be made.
One of the most important methods of bonding analysis however was the Energy Decomposition Analysis with Natural Orbitals for Chemical Valence (EDA-NOCV), which allowed subsequent to an inspection of deformation densities the break-up of the orbital interaction term into separate contributions during bond formation, thereby providing a deeper understanding of the respective chemical bond.
The next section dealt with organozinc rich ruthenium compounds and their homologues [Co*M(YR)5] or [M(YR)10] (M=Fe–Os; Y=Zn–Hg; R= organyl group), which were prepared by Mariusz Molon and Thomas Cadenbach (Ruhr University Bochum). At the first glance these compounds could exhibit cluster characteristics, because of their high coordination numbers. These systems also fulfill the 18 VE rule if the YR ligands are interpreted as one-electron donors, but they do not possess the required number of CVE to form metal clusters. This image was reinforced by the fact that tangential interactions, which could be detected with the help of a topological analysis of the electron density, were mainly found in particularly electron rich and diffuse mercury ligands only. The inspection of partial charges indicates that during the EDA-NOCV the central metal atom should be an M2+ cation which implies that once more closed shell fragments are interacting via donor-acceptor bonds with each other. And again the involved ligand orbitals assume the shape of s-, p- and d-like molecular orbitals (MOs), which interact with the atomic orbitals of the coordinated group 8 metals. The trends of these bond strengths can be explained by the group characteristics of the involved atom types.The various organyl metal ligands exhibit very similar bonding characteristics. This fact could be the reason why the structures obtained in experiments are not homoleptic. The Cp* ligand, which is directly coordinated to the M atom has an extremely strong bond in comparison to the YR groups. Therefore it could be considered as an inert ligand in this case.
In the last section (cAAC)-stabilized coinage metal(0)-complexes of the (cAAC)2 Z, (cAAC)2Z(+), and (cAAC)2Z2 type with Z = Cu, Ag and Au were investigated. ese complexes were synthesized by David Weinberger (University of California, San Diego) and Herbert W. Roesky et al. (Georg-August University Göttingen). They are linear carbene coordination compounds in which the neutral (cAAC)2Z complex has a doublet ground state. Analyses applying the NBO, QTAIM, and EDA-NOCV methods show that the Z–C bond strength is highest in neutral compounds and lowest in di-metal systems. It can be shown that the influence of relativistic effects is responsible for this trend.