Acta Crystallographica Section B

Structural Science

Volume 58, Part 3 Number 2 (June 2002)

research papers

Acta Cryst. (2002). B58, 463-470 [ doi:10.1107/S0108768102002410 ]

Nucleophilic addition reaction in coordinated non-linear pseudohalides: experimental charge density analysis in trans-bis(cyanamidonitrato-N:O)bis-(imidazole-N³)copper(II) complex

J. Kozísek, N. K. Hansen and H. Fuess

Abstract: The experimental crystal charge density of the complex [Cu(iz)₂(can)₂] (I) (where iz = imidazole, can = cyanamidonitrate anion), [Cu(C₃H₄N₂)₂(CN₃O₂)₂], crystallizing in centrosymmetric space group P2₁/c, has been determined at 120.0 (2) K, using Hansen and Coppens' aspherical atom multipole model. The central Cu atom is pseudo-octahedrally coordinated by four N atoms in an equatorial plane [two from iz molecules, 1.9660 (4) Å, two from can anions, 2.0273 (5) Å] and by two O atoms (from two adjacent molecules) in axial positions [2.5566 (5) Å]. The d-orbital population analysis shows that the oxidation state for the copper is +2 with the d⁹ configuration and that there is a hole in the $d_{x^2 - y^2}$ orbital, located in the plane of the four nitrogen-ligating atoms. Electron lone pairs of these four N atoms are pointing into these regions of depleted electron density. In accordance with the medium-length Cu-O axial bonds, the d_z² orbital is almost fully occupied [1.79 (3) electrons]. The non-bonding d_xy orbital is fully occupied [2.05 (3) electrons]. Analysis of the atomic charges does indicate a deficit of +0.35 (4) e^- on the nitrile carbon, which provides evidence of [beta] -carbon activation for a possible nucleophilic addition reaction. This reaction has been indirectly confirmed experimentally.

Keywords: nucleophilic addition; charge density analysis; aspherical atom multipole model.