Experimental electron density of the complex carbides Sc3[Fe(C2)(2)] and Sc3[Co(C2)(2)]

The nature of chemical bonding in the complex carbides Sc3[Fe(C2)2] (1) and Sc3[Co(C2)2] (2) has been explored by combined experimental and theoretical charge density studies. The structures of these organometallic carbides contain one-dimensional infinite TC4 (T = Fe, Co) ribbons embedded in a scandium matrix. Bonding in 1 and 2 was studied experimentally by multipolar refinements based on high-resolution X-ray data and compared to scalar-relativistic electronic structure calculations using the augmented spherical wave method. Besides substantial covalent T-C bonding within the TC4 ribbons, one also observes discrete Sc-C bonds of noticeable covalent character. Furthermore, our study highlights that even tiny differences in the electronic band structure of solids might be faithfully recovered in the properties of the Laplacian of the experimental electron density. In our case, the increase of the Fermi level in the organometallic Co(d9) carbide 2 relative to its isotypic Fe(d8) species 1 is reflected in the charge density picture by a significant change in the polarization pattern displayed by valence shell charge concentrations of the transition metal centers in the TC4 units. Hence, precise high-resolution X-ray diffraction data provide a reliable tool to discriminate and analyze the local electronic structures of isotypic solids, even in the presence of a severe coloring problem (Z(Fe)/Z(Co) = 26/27).