Herein we report the transformation of [Au₂₅(SR)₁₈]⁰ into Au₂₈(SR)₂₁ induced by ligand exchange reaction. In contrast to other reported cluster transformations, which proceed at elevated temperature and large excess of incoming ligands, the transformation reported here occurs under mild conditions (room temperature, very low thiol excess) with a chiral ligand. A difference of one methyl group between incoming and leaving thiol is sufficient to induce the transformation. To the best of our knowledge the Au₂₈(SR)₂₁ cluster has not been isolated before.