The reaction between the formamidine ligand PriN=CHNHPri and the activated cluster Os3(CO)10(MeCN)2 has been studied. A rapid reaction is observed at room temperature, yielding the hydride clusters HOs3(CO)9[μ-OCNPriC(H)NPri] and HOs3(CO)10[μ-NPriC(H)NPri] as the principal products. The spectroscopic data and X-ray diffraction structures of those formamidinate-substituted clusters will be present. The thermal reactivity of the clusters has been investigated, with the face-capped cluster HOs3(CO)9[μ-NPriC(H)NPri] found as the sole observable product. The relationship between these three clusters has been established by kinetic studies, the results of which will be discussed.