PUB - Publikationen an der Universität Bielefeld

The mono- and bidentate formation of the amidinate ligands toward the main group elements silicon, germanium, tin, and lead is studied with quantum chemical methods. In accordance with the experimental investigations the bisamidinate complexes of the higher element homologues Pb and Sn in their divalent oxidation states adopt C-2 symmetrical structures, with a distorted trigonal bipyramidal (Psi-tpb) arrangement. The formation of a pseudotrigonal bipyramid is less favorable with the lighter elements Si and Ge; the resulting structures easily undergo Jahn-Teller distortion to C-1 symmetrical structures in which the axial bonds become unsymmetrical. A structural rearrangement from C-2 to C-2 upsilon symmetry is even feasible and causes a facile degenerate rearrangement in which the nitrogens in the amidinate ligands become positionally equivalent. The effect of substituents on these different structures is discussed. An analysis of the electron distributions (natural bond orbital populations, Laplacian of the charge densities) establishes for these compounds a strongly positive charged central atom (M = Si, Ge, Sn, Pb), weakly chelated by the surrounding amidinate ligands. The equilibrium between chelated and covalent bound structures is discussed on the basis of a bond stretch formalism. Structural predictions are also made for ligands isoelectronic to the amidinate anion [HNCHNH](-), (e.g., [HPCHPH](-), [HNSiHNH](-), and [OCHO]-). Accordingly, bisamidinate-like structures are promoted with increasing difference in electronegativity between the central atom (Si, Ge, etc.) and the neighboring chelating atom (P < N < O). The design of structures with a tetrahedral environment of the central atom in which the nonbonding s-orbital is stereochemically not active is discussed. Such conformations possess large open shell character as tested by the electron correlated methods and MCSCF calculations and are promoted by chelating ligands with an inherent weak pi-allylic system. The matter is tested for silicon as central atom for the homologous bisamidinate complex.