Bonding properties related with Chiral discrimination in dinuclear metal complexes of group 10

"A series of neutral dinuclear homo- and heterochiral complexes formed by hydrazine (a molecule with axial chirality), a metal atom of group 10 (Ni, Pd, Pt), and halogen atoms (F, Cl, Br, 1) was studied by computational methods including density functional theory. In the optimized structures the heterochiral complexes were always found to be energetically favored. The chiral discrimination depends on the contributions coming from the metal atoms and the halogen anions attached to them. Moreover, the relative energies of the optimized complexes are correlated with the electronegativity of the metals and the halogens as well as with the relative distances of the bond critical points in the metal-nitrogen bonds. Also the smaller deviation of the ligand-metal-ligand angles from the ideal value of 90 degrees corresponds to the heterochiral complexes. An analysis of the bonding properties reveals a complex mechanism for the behavior of the electron lone pairs of the nitrogen and halogen atoms. The electronic properties of the optimized complexes were studied by NBO (Natural Bond Orbital) and QTAIM (Quantum Theory of Atoms in Molecules) methods. An NBO analysis of the energy differences between the stabilizing energies related to the interactions between an electron lone pair of the coordinated nitrogen atom and the opposite M-X antibonding orbital explain the differences of the total energies between the diastereomeric heterochiral and homochiral complexes. The stabilizing energies have systematically more negative values in the heterochiral complexes; this can be interpreteted as a measure of the trans influence. Small energy values related to the cis influence were found mainly in the nickel complexes. The M-N bonds in the heterochiral complexes are more polar with a higher bond order than that in the homochiral ones, as shown by QTAIM methods. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)"