Here, we report the first study focused on atom-by-atom substitution of Fe and Ni to the core of a well-defined cobalt sulfide superatom ligated with triethylphosphine,[Co5MS8(PEt3)6]+ (M=Fe, Ni). Electrospray ionization mass spectrometry confirms the substitution of 1-6 Fe atoms with the single Fe-substituted cluster being the dominant species. The Fe-substituted clusters oxidize in solution to generate dicationic species. In contrast, only a single Ni-substituted cluster is observed, which remains stable as a singly charged species. Collision-induced dissociation experiments indicate the reduced stability of the [Co5FeS8(PEt3)]+ towards ligand loss in comparison with the unsubstituted and Ni-substituted counterparts. Density functional theory calculations provide insights into the effect of metal atom substitution on the electronic structures of the clusters. Our results indicate that Fe and Ni have a different impact on the electronic structure, optical and magnetic properties, as well as ligand-core interaction of [Co6S8(PEt3)6]. This study extends the atom-by-atom substitution strategy to the metal chalcogenide superatoms providing a direct path toward designing novel atomically precise core-tailored superatoms.