[Os5C(CO)14]2- was synthesized on the surface of MgO by reductive carbonylation of adsorbed Os3(CO)12 at 548 K and 1 bar. The supported species were characterized by infrared (IR), 13C NMR, and extended X-ray absorption fine structure (EXAFS) spectroscopies. The IR and EXAFS data are consistent with the presence of [Os5C(CO)14]2-, formed in a yield of about 65%, along with smaller osmium carbonyl clusters. As the supported clusters were decarbonylated in flowing He or H2, they were characterized by IR and EXAFS spectroscopies, which indicate that the decarbonylation was complete after each treatment at 573 K. The EXAFS data characterizing the sample treated in He determine an Os-Os first-shell coordination number of 3.4, matching that of [Os5C(CO)14]2- and indicating that the Os5C frame was retained after decarbonylation in He. Treatment of MgO-supported [Os5C(CO)14]2- in H2 at 573 K resulted in the formation of aggregated osmium clusters larger than Os5C. The catalytic activity of Os5C for toluene hydrogenation was found to be an order of magnitude less than that of the aggregated osmium clusters, which are metallic in character.