Beiträge zur Bindungstheorie und Aufklärung von Reaktionsmechanismen : Quantenchemische Untersuchungen.

Quantumchemical investigations employing several Density Functionals conclude that the compound (L:)2SiCl2 has a biradical singlet ground state. This compound results from the reaction of the donor acceptor complex (L':)SiCl2} which is based on the N-heteorcyclic carbene with a cyclic alkyl amino carbene. The stability of (L:)2SiCl2 is determined through covalent electron-sharing C-Si-bonds, with one electron originating from carbon and silicon. The remaining electrons at the carbon atoms may couple parallel (triplet) or anti-parallel (open shell singlet) and result in the biradical nature of (L:)2SiCl2. Under reductive conditions the silylone (L:)2Si has been derived from (L:)2SiCl2. This compound is the first example for a carbene stabilized silicon in the oxidation state zero. Its ground state is a closed shell singlet which can, due to its very small HOMO LUMO gap, easily be excited to a biradical state. Very high first and second proton affinities, as well as the shape of the highest lying molecular orbitals characterize (L:)2Si as a genuine silylone. With a similar synthetic strategy a two-coordinate zinc compound (L:)2Zn may be obtained. The ground state is a mixture of open and closed shell singlet, hence it has some biradicaloid character. The C-Zn-bonds can be described as covalent, electron-sharing bonds which are strongly polarized towards the carbon. Its biradical nature originates from the remaining electrons at the carben carbon, which couple in an anti-forromagnetic manner. The second topic of this dissertation deals with pincer complexes of electron poor metals as central atom. The ligand of the investigated complexes, bis-(iminophosphorano)-methandiide, acts as a tridentate eight electron donor. The central C-M-bond of the complexes of group-4-elements (Ti, Zr, Hf) may be seperated into sigma- and pi-type interaction. This separation is no longer possible with the compounds of the alkaline earth metals (Ca, Sr, Ba). The two lone pairs at the carbon atom are both of p- or pi-type, perpendicular to each other, and rotated towards the metal center. In the dimerized compounds of group-2-elements (Ca, Sr, Ba), the ligand again acts as an eight electron donor, but neither between the two metal atoms nor between the carbon atoms bonds are presen are present. The ligand itself, which originated from deprotonation of bis-(iminophosphorano)-methane, has to be described as a carbone, as the quantumchemical investigations prove. The carbon has a formal oxidation state of zero and is stabilized through two donor acceptor interactions from the iminophosphorano groups. The results of the calculations for the pincer complexes of Ti, Zr, Hf are in good agreement with experimentally obtained structure parameters. The central M-C-bonds of these compounds are overestimated at the BP86 level of theory and hence are too short. The obtained qualitative statements about the bonding situation have to be supported by further application of different functionals. In the third part of this dissertation the beginning of the polymerization reaction of guanidine hydrochloride and diethylene triamine is investigated. In this reaction polymer compounds with a cyclic repeating unit is formed, while the expected linear product can not be found. The discovered reaction mechanism supports the experimentally obtained results. The activation barrier for the ring closure is in all cases lower in energy (ca. 10 kcal/mol) than the barrier for the back reaction. The process is catalyzed by the generated ammonia or the triamines. These molecules act as a proton transfer agent. The competing reactions have also been investigated and yield higher barriers than for the ring closure (ca. 4 to 15 kcal/mol). Hence the back reaction is more likely to occur. The thermodynamical estimations of the reactions of guanidine and dipropylene triamine or ethylene propylene triamin are not in agreement of the experimental results. The results of the calculations suggest that six membered ring structures are thermodynamically most stable. The formation of these polymer products may strongly be kinetically hindered, so that the experimentally observed products are formed first. A further investigation of the reaction mechanisms is necessary to obtain certainty.