X-Ray–neutron diffraction study of the electron-density distribution in trans-tetraaminedinitronickel(II) at 9 K: transition-metal bonding and topological analysis

Abstract

A quantitative description of transition-metal bonding has been obtained through combined analysis of 9(1) K X-ray and 13(1) K time-of-flight neutron diffraction data. It is shown that a simple valence-orbital model is too crude an approximation adequately to describe the electron-density distribution of Ni(ND₃)₄(NO₂)₂. To exhaust more fully the information present in the very-low-temperature diffraction data, a more flexible electron-density model was used. Quantitative measures describing the bonding in the complex have been achieved through topological analysis of the derived static model density. To study the effects of co-ordination and intermolecular interactions, comparisons were made with good-quality wavefunctions calculated for free nitrite and ammonium ions. Both ligands appear co-ordinated through predominantly electrostatic interactions. Contrary to previous studies of Ni(ND₃)₄(NO₂)₂, the topological analysis revealed that the metal–ligand interactions, besides cylindrical σ contributions, also have non-cylindrical π contributions to the covalent part of the bonding. Plots of the Laplacian of the electron density were used to locate regions of charge concentration and charge depletion in the valence regions of the atoms in the molecule. For all atoms, maxima in the valence-shell charge concentration are found in accord with the simple Lewis electron-pair concept of bonded and non-bonded charge concentrations. The study demonstrates that X-ray diffraction data measured carefully at very low temperatures have sufficient precision to allow for a reliable and detailed topological analysis of transition-metal electron-density distributions.

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