Os 5 C(CO) 14 ] 2was synthesized in 65% yield on the surface of MgO by treatment of adsorbed [Os 3 (CO) 12 ] in CO. High-resolution transmission electron microscopy (HRTEM) indicated that the nanoclusters were nearly monodisperse and in the size range consistent with [Os 5 C(CO) 14 ] 2-. The supported [Os 5 C(CO) 14 ] 2was decarbonylated in helium, giving supported nanoclusters that are well approximated as Os 5 C, as shown by HRTEM and EXAFS data. Decarbonylation in H 2 gave larger clusters of osmium, as shown by HRTEM.