Topological Analysis of the Electron Density Distribution of Bis(diiminosuccinonitrilo)nickel, Ni(C<INF>4</INF>N<INF>4</INF>H<INF>2</INF>)<INF>2</INF>:  Comparison between Experiment and Theory

A quantitative description of chemical bonds in bis(diiminosuccinonitrilo)nickel, Ni(disn)<INF>2</INF>, is made in terms of topological properties of electron densities. These properties are obtained both from an X-ray diffraction experiment and from molecular orbital calculations. The asphericity in electron density around the Ni ion is surely observable from the Laplacian of the electron density with density accumulation in the d<INF>π</INF> direction but density depletion along the d<INF>σ</INF> (Ni−N) direction. On the basis of the topological properties at bond critical points, the bonding between Ni and the imino nitrogen atom is classified as mainly a closed-shell interaction but with some covalent character. The bonds within the ligand, disn, are all shared interactions, and the bond order is reflected clearly from the density at the critical point, ρ(<BO>r</BO><INF>c</INF>). The π-delocalization of the molecule is precisely indicated by the bond ellipticity and is illustrated by Fermi-hole distribution. Atom domains in the molecule are demonstrated. Molecular electrostatic potential is derived both from experiment and from MO calculations. For all the properties, the agreement between experiment and theory is reasonable.