Structures of medium-sized ruthenium clusters: the octahedral motif.

We describe the first direct structural characterization of medium-sized ruthenium clusters (Ru19 (-) , Ru28 (-) , Ru38 (-) , and Ru44 (-) ) by using a combination of trapped ion electron diffraction and density functional theory. We find close-packed structures based on octahedral geometries: Ru19 (-) and Ru44 (-) are closed-shell octahedra whereas Ru28 (-) is a double octahedron. In the case of Ru38 (-) , instead of a truncated octahedron we obtain evidence for lower symmetry structures containing a reentrant surface.