Bond Characterization of Metal Squarate Complexes [MII(C4O4)(H2O)4; M = Fe, Co, Ni, Zn

Abstract

Tetraaqua metal squarate complexes, M(C4O4)(H2O)4, (M = Fe, Co, Ni, Zn), are known to have a polymeric chain structure with C4O42- as a bridge (μ-2) ligand between two metal ions in the trans position. Each metal ion is bonded to two C4O42- and four water molecules. They are all isostructural with space group C2/c. A complete Ewald sphere of data is measured at 120 K up to 2θ of 100−120° using Mo Kα radiation for each complex. Such carefully measured intensities are used to investigate the detailed electron density distribution in order to understand the chemical bonding and the d-orbital splitting of the metal ions subjected in such a ligand field. Results on the electron density distribution will be presented in the form of deformation density and of Laplacian maps. Deformation density will be presented in terms of experimental Δρx-x, Δρm-a (multipole model), and of theoretical Δρ derived from the HF and DFT calculations. The interesting bent bond feature on the four-membered ring ligand C4O42- is explicitly demonstrated by the deformation density distribution and the bond path of the cyclic carbon−carbon bond. The asphericity in electron density distribution around the metal ion is also clearly illustrated in these compounds both in the deformation density and in Laplacian of the density. The comparison on the series of 3d-transition metal complexes will be made not only by the deformation density distribution and by the Laplacian of the density but also by the d-orbital population and by the associated topological properties at the bond critical point. The total number of d-electrons from the experiments are 6.05, 6.88, 7.89, and 8.40, respectively, for Fe(II), Co(II), Ni(II), and Zn(II) ions in these compounds. A comparison between experiment and theory is made for the Ni complex.