The core of X@Ag54 consisted of only Ag and S and formed a central cavity, as observed in previously reported [S@Ag50S12(S t Bu)20](TFA)4. In the previously reported cluster, all the Ag atoms are in the +1 oxidation state, giving rise to a single sharp peak in both the Ag 3d3/2 and 3d5/2 regions of its XPS spectrum. In contrast, broad peaks were observed in the Ag 3d5/2 and 3d3/2 regions of the XPS spectra of X@Ag54 (Figure ). These peaks were fitted with two functions, and the position of the peak on the high-energy side was close to the position of the peak corresponding to Ag2S or AgO (Figures and, Table ). These observations indicates that Ag in X@Ag54 is in a mixed-valence state, rather than the single-valence state of Ag in [S@Ag50S12(S t Bu)20](TFA)4. In fact, there have been several reports on Ag existing in mixed-valence states. For example, bulk silver monoxide (AgO), which can be described as Ag2O•Ag2O3, contains Ag in a mixed-valence state of Ag I and Ag III . Furthermore, Cu atoms in Cu-chalcogen clusters with geometric structures similar to those of X@Ag54, such as [S@Cu54S12O6(S t Bu)20(SO3 t Bu)12] and [Cu50S12(S t Bu)20(TFA)12], also exist in a mixed-valence state of Cu I and Cu II . Unlike [S@Ag50S12(S t Bu)20](TFA)4, X@Ag54 contained two types of ligands (S t Bu and SO3 t Bu), and this can be considered to cause the mixed-valence state of Ag.