Bonding properties of amidinate complexes of the group 14 elements silicon, germanium, tin, and lead in their divalent and tetravalent oxidation states

The mono- and bidentate formation of the amidinate ligands toward the main group elements silicon, germanium, tin, and lead is studied with quantum ch emical methods. In accordance with the experimental investigations the bisa midinate complexes of the higher element homologues Pb and Sn in their diva lent oxidation states adopt C-2 symmetrical structures, with a distorted tr igonal bipyramidal (Psi-tpb) arrangement. The formation of a pseudotrigonal bipyramid is less favorable with the lighter elements Si and Ge; the resul ting structures easily undergo Jahn-Teller distortion to C-1 symmetrical st ructures in which the axial bonds become unsymmetrical. A structural rearra ngement from C-2 to C-2 upsilon symmetry is even feasible and causes a faci le degenerate rearrangement in which the nitrogens in the amidinate ligands become positionally equivalent. The effect of substituents on these differ ent structures is discussed. An analysis of the electron distributions (nat ural bond orbital populations, Laplacian of the charge densities) establish es for these compounds a strongly positive charged central atom (M = Si, Ge , Sn, Pb), weakly chelated by the surrounding amidinate ligands. The equili brium between chelated and covalent bound structures is discussed on the ba sis of a bond stretch formalism. Structural predictions are also made for l igands 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 centr al 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 ele ctron correlated methods and MCSCF calculations and are promoted by chelati ng ligands with an inherent weak pi-allylic system. The matter is tested fo r silicon as central atom for the homologous bisamidinate complex.