Experimental Determination of the Electron Density Topology in a Non-centrosymmetric Transition Metal Complex:  [Ni(H3L)][NO3][PF6] [H3L = N,N‘,N‘‘-Tris(2-hydroxy-3-methylbutyl)-1,4,7-triazacyclononane]

We have determined the experimental charge density of the title compound, a large pendant arm macrocyclic complex, by fitting a multipole model to high-resolution X-ray diffraction data. We find redistribution of the charge density from the ligating nitrogen and oxygen atoms to the central nickel ion which considerably reduces the charge on the metal. While the d-orbital populations are in agreement with ligand-field theory, the charge density of the metal shows diffuse features which are ascribed to metal 4s and 4p participation in the metal−ligand bonding. Additionally, the trigonal-planar arrangement of critical points in −∇2ρ(r) within the valence shell charge concentration (VSCC) of the ligating oxygen atom provides evidence for metal−oxygen π-bonding.