Quantifying Chemical Differences of Cl– and Br– Ligands: A Comparative X-ray Electron Density Study of Two Isostructural Co(II) Single Molecule Magnets

Abstract The magnetic properties of single molecule magnets (SMMs) relate to details of the metal–ligand chemical bonding. Halide ligands are typical structural entities in SMMs and are often interchanged assuming chemical transferability. However, the chemical bonding of the halides is different, and it is of high interest to understand how these differences affect the properties of SMMs. Here, two isostructural pentacoordinate cobalt complexes, [Co(L)Cl2] (1) and [Co(L)Br2] (2) with L = 2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)-pyridine, are investigated through 25 K single crystal synchrotron X-ray diffraction (XRD) measurements with subsequent multipole modeling to derive experimental electron densities. Topological analysis of the electron densities reveal closed-shell interactions between Co and the halides and the nitrogen donors of L. The Co–Cl bonding resembles the Co–N bonds, with very similar electron density and Laplacian in the bond critical points, while the Co–Br bonding is weaker and more ionic. Estimated experimental d-orbital populations reveal an ordering of (dxy,dxz) < dz2 < dyz < dx2–y2 for 1 and dxy < dxz < dx2–y2 < dyz < dz2 for 2. The reversal of dz2 and dx2–y2 between 1 and 2 reflects a stronger interaction from Co to the equatorial Cl compared to the axial Cl in 1, but vice versa in 2.