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 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 C2 symmetrical structures, with a distorted trigonal bipyramidal (Ψ-tpb) arrangement. The formation of a pseudo-trigonal bipyramid is less favorable with the lighter elements Si and Ge; the resulting structures easily undergo Jahn−Teller distortion to C1 symmetrical structures in which the axial bonds become unsymmetrical. A structural rearrangement from C2 to C2v 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 de...