A discrete nanocage of core-shell design, in which carboxylic acid groups were tethered to the core and silanol to the shell interior, was found to react with Co-2(CO)(8) to form and stabilize a Co(I)-CO species. The singular CO stretching band of this new Co species at 1958 cm(-1) and its magnetic susceptibility were consistent with Co(I) compounds. When exposed to O-2, it transformed from an EPR inactive to an EPR active species indicative of oxidation of Co(I) to Co(II) with the formation of H2O2. It could be oxidized also by organoazide or water. Its residence in the nanocage interior was confirmed by size selectivity in the oxidation process and the fact that the entrapped Co species could not be accessed by an electrode.