Nearly uniform decaosmium clusters supported on MgO: Characterization by x-ray absorption spectroscopy and scanning transmission electron microscopy

Samples containing small, nearly uniform clusters of a heavy metal, Os, were prepared on a high-area porous support consisting of light atoms, MgO, to provide an opportunity for a critical assessment of estimates of cluster size determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Supported carbonyl clusters approximated as decaosmium were prepared by reductive carbonylation of adsorbed Os ₃(CO) ₁₂ at 548 K and 1 bar. Infrared (IR) spectra of the clusters resemble those attributed in earlier work to supported clusters similar to [Os ₁₀C(CO) ₂₄] ^2-, consistent with the EXAFS data. The spectra indicate a molar yield of decaosmium carbonyl clusters of about 65-70%. As these clusters were treated in flowing H ₂, they were partially decarbonylated, as shown by IR and EXAFS spectra. The rms (root-mean-square) radii of the undecarbonylated and partially decarbonylated clusters were found by HAADF-STEM to be 3.11 ( 0.09 and 3.06 ( 0.05 Å, respectively, and the close agreement between these values is consistent with the inference that the cluster frame was essentially the same in each. The average rms radius of the undecarbonylated clusters, estimated on the basis of EXAFS data, was 2.94 ( 0.07 Å, calculated on the basis of the assumption that the osmium frame matched that of [Os ₁₀C(CO) ₂₄] ^2-. EXAFS analysis of the data characterizing the partially decarbonylated sample, aided by the STEM results, showed, consistent with the STEM results, that the partial decarbonylation did not lead to a significant change in the rms radius of the metal frame.,