Ligand-free osmium clusters supported on MgO.: A density functional study

The interactions of Os-4, Os-5, and Os5C clusters with various sites of a MgO(001) support were investigated theoretically with the aid of a scalar-relativistic density functional cluster model method. Adsorption geometries of C-4v clusters centered above a magnesium cation and the Os atoms oriented either to the nearest surface oxygen anions (A) or between them (B) were considered. The influence of surface V-s and V-8(2-) defects on the adsorption of the clusters was also investigated. The calculated base Os-Os distances in supported Os-5 and Os5C square-pyramidal clusters are at most 0.1 Angstrom longer (2.5-2.6 Angstrom) than the values calculated for the corresponding free osmium clusters but about 0.4 Angstrom (or more) shorter than the values determined by EXAFS spectroscopy for MgO-powder-supported clusters formed by decarbonylation of [Os5C(CO)(14)](2-) and shown to retain the Os5C frame. The experimental Os-Os distances characterizing the supported clusters are close to the experimental and calculated bond lengths for coordinatively saturated osmium carbonyl clusters; this result favors the suggestion that the supported clusters characterized by EXAFS spectroscopy were not entirely ligand-free. Calculated interaction strengths of the osmium clusters with the MgO(001) support range from nonbonding (defect-free site B when the basal Os atoms are aligned between the nearest O anions) to very weak (0.6 eV for Os5C at defect-free site A when the basal Os atoms are aligned with the nearest O anions) to weak (similar to 2 eV for pure Os clusters at defect-free site A) to rather strong (similar to 9 eV for V-s defect site A). The models reported here are inferred to be too simplified to capture all the pertinent structural details of MgO-powder-supported osmium clusters, but they are sufficient to indicate a significant role of defect sites in the adsorption of supported osmium clusters and, we infer, other transition metal clusters.