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 M gO(001) support were investigated theoretically with the aid of a scalar-re lativistic density functional cluster model method. Adsorption geometries o f 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 c onsidered. The influence of surface V-s and V-8(2-) defects on the adsorpti on of the clusters was also investigated. The calculated base Os-Os distanc es in supported Os-5 and Os5C square-pyramidal clusters are at most 0.1 Ang strom longer (2.5-2.6 Angstrom) than the values calculated for the correspo nding free osmium clusters but about 0.4 Angstrom (or more) shorter than th e values determined by EXAFS spectroscopy for MgO-powder-supported clusters formed by decarbonylation of [Os5C(CO)(14)](2-) and shown to retain the Os 5C frame. The experimental Os-Os distances characterizing the supported clu sters are close to the experimental and calculated bond lengths for coordin atively saturated osmium carbonyl clusters; this result favors the suggesti on that the supported clusters characterized by EXAFS spectroscopy were not entirely ligand-free. Calculated interaction strengths of the osmium clust ers with the MgO(001) support range from nonbonding (defect-free site B whe n 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 a t 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 cluster s, but they are sufficient to indicate a significant role of defect sites i n the adsorption of supported osmium clusters and, we infer, other transiti on metal clusters.