In this study, we successfully synthesized a pair of X@Ag54 clusters (X = S or I) protected by two types of ligands, thiolate and sulfonate, by adding Ag + and a catalytic amount of Cu 2+ to the reaction system. No significant difference was observed between the two X@Ag54 clusters (X = S or I) in terms of geometric structure and stability against degradation. However, notable differences were observed in their PL and optical absorption. We concluded that the differences in their optical properties are derived from the heavy atom effect. This effect enhances the oscillator strength of transitions from the spin-forbidden excited singlet state to the triplet excited state, as well as transitions from the ground singlet state to the triplet excited state. This study clearly demonstrated that incorporating heavy atoms into Ag-S clusters significantly enhances their phosphorescence quantum yield, similar to organic fluorescent dyes. These findings are expected to provide clear design guidelines for developing metal clusters as room-temperature phosphorescent materials and triplet sensitizers.