Alumina-supported triosmium clusters and ensembles: Characterization by high-resolution transmission electron microscopy

Structurally defined supported osmium clusters [HOs₃(CO)₁₀OAl<] were prepared by the reaction of [Os₃(CO)₁₂] with the -OH groups of γ-Al₂O₃. The supported clusters were heated to 200 °C in argon and then reduced in hydrogen at 400 °C, being characterized at different stages of the treatment by high-resolution transmission electron microscopy. After the sample had been heated to 200 °C, scattering centers about 6 Å in diameter were evident in the micrographs, their size being consistent with the expected geometry of three-atom ensembles of Os(II) complexes formed by cluster breakup. The ensembles were remarkably stable, and even prolonged exposure of the same area to the 100 keV electron beam caused neither disintegration nor agglomeration. After reduction of the osmium in hydrogen at 400 °C, aggregates of similar size were observed. In contrast to the ensembles of Os(II) complexes, the aggregates seemed to disintegrate slowly under the impact of the electron beam. The difference in stability of the ensembles and the reduced osmium aggregates is attributed to the difference between the strong ionic bonds in the former sample and the weaker interaction between the reduced metal and the Al₂O₃ surface.