These geometric structures of X@Ag54 are similar to those of previously reported clusters, such as S@Ag40.13Cu13.87S18(S t Bu)20(SO3 t Bu)12 and S@Cu54S12O6(S t Bu)20(SO3 t Bu)12. In the two previously reported clusters, the M42(S t Bu)20(SO3 t Bu)12 (M = Ag or Cu) shell, which has Th symmetry, also covers the core. However, there are two major differences between these two previously reported clusters and X@Ag54: 1) in the previously reported clusters, Ag and Cu, or only Cu, are included as metals, whereas in X@Ag54, only Ag is present as the metal of the clusters; 2) the previously reported clusters have two fewer chalcogenide ions (S 2-or O 2-) other than the central ion (18 chalcogenide ions) than X@Ag54 (20 sulfide ions). As a result, the diameter of the cavity of the M12S8 core of X@Ag54 (approximately 5.04 and 5.13 Å for S@Ag54 and I@Ag54, respectively; Figures and), is greater than that of the two previously reported clusters (4.94 Å and 4.88 Å). For this reason, both S 2-(ionic radius = 1.84 Å) and I -(ionic radius = 2.20 Å), were stably encapsulated inside the {M12S8} core of X@Ag54. Formation mechanism X@Ag54 contains 20 S atoms. These S atoms are considered to be produced through the reaction between Ag-S t Bu and Ag + : this reaction produces Ag2S. In this study, although only t BuSH was added as the ligand during the synthesis, in-situ produced SO3 t Bu was also included as a ligand in X@Ag54, as confirmed by SC-XRD (Figure ), 1 H NMR (Figure ), and ESI-mass spectroscopy (Figure ), in addition to X-ray photoelectron spectroscopy (XPS) (Figures and) and Fourier-transform infrared (FT-IR) absorption spectroscopy (Figure ), of the products. The catalytic oxidation of thiols by Cu 2+ (Scheme S1) is responsible for the formation of these SO3 t Bu ligands. In this reaction, Cu 2+ coordinates with RSH, which reduces Cu (Cu + ) and generates the thiyl radical (RS•). RS• reacts with oxygen molecular (O2) and unreacted RSH in the system to produce RSO3H (Figure ). After coordinating with RSH, Cu + is oxidized to Cu 2+ by O2, returning to its original state. As these reactions proceed, SO3 t Bu is formed in the reaction solvent and coordinates with the metal cluster (Scheme S1). In these reactions, the coordination and reduction of Cu 2+ and formation of RSO3H from RS• proceed with rate constants of approximately 10 8-10 M -1 s -1 , which is very fast. Moreover, these reactions are faster than the oxidation of Cu + to Cu 2+ by O2, which becomes the rate-limiting step. Indeed, when Cu(NO3)2 and t BuSH were mixed, the blue color of the solution immediately disappeared owing to the reduction of Cu 2+ to colorless Cu + , whereas this blue color took 2 days to reappear as Cu + reverted to Cu 2+ (Figure ). However, when Ag + coexisted with Cu 2+ and t BuSH, the recovery of Cu 2+ from Cu + occurred more rapidly (Figure ), with the disappearance of t BuSH confirmed by electron spin resonance (ESR) spectroscopy of the reaction solution. These results demonstrate that the presence of Ag + accelerates the rate-limiting step of RSH oxidation by Cu 2+ , thereby accelerating the formation of RSO3H.