Ruthenium-arene complexes bearing the 1,3,5-triaza-7-phosphatricyclo-[3.3.1.1]decane ligand, namely, RAPTA compounds, are widely investigated as potential antimetastatic agents for cancer therapy. Although much evidence points toward covalent binding with essential protein biomolecules as the key determinant of their activity, the exact biological targets of RAPTA remain elusive. To address this current gap in understanding, RAPTA compounds derivatized with acetal groups via a pendant chain to the arene ligand were developed as a functional probe for selective postlabeling and tagging of covalent RAPTA adducts in cancer cells.