A novel platinum(II) photocaged complex called [Pt(cage)] has been prepared and characterized by X-ray crystallography. The complex contains a photolabile nitrophenyl group incorporated into the backbone of a tetradentate ligand that contains two pyridyl and two amide nitrogen donor sites. The intact complex is unreactive toward ligand-exchange reactions until activation with UV light cleaves the ligand backbone, releasing a PtII complex that more readily exchanges its ligands, as verified by reaction with a methioninc-containing peptide. [Pt(cage)] is non-toxic to MCF-7 cells in the dark, whereas brief UV exposure induces cell death of human breast cancer MCF-7 cells at a level approaching that of cisplatin. By using light to alter the coordination chemistry around the metal center, [Pt(cage)] represents a new strategy for potentially delivering metal-based drugs in a site and time specific manner. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.