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

The interactions of Os₄, Os₅, 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_s^2- defects on the adsorption of the clusters was also investigated. The calculated base Os-Os distances in supported Os₅ 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 (approx. 2 eV for pure Os clusters at defect-free site A) to rather strong (approx. 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.