Sulfur-metal complexes, containing only a few atoms, can open new, highly efficient pathways for transport of metal atoms on surfaces. For example, they can accelerate changes in the shape and size of morphological features, such as two-dimensional nanoclusters, over time. In this study we perform STM under conditions that are designed to specifically isolate such complexes. We find a new, unexpected S-Cu complex on the Cu(111) surface, which we identify as Cu₂S₃. We propose that Cu₂S₃ enhances mass transport in this system, which contradicts a previous proposal based on Cu₃S₃. We analyze bonding within these Cu-S complexes, identifying a principle for stabilization of sulfur complexes on coinage metal surfaces.