the invention is directed to providing a compound for treating cancer , the compound comprising a metal complex having predetermined characteristics , and which may be incorporated into a polymeric nanoparticle or other delivery system for delivering the metal complex for action on tumor cells . the metal complex for treating cancer wherein the metal complex is a silver salt , a silver macrocyclic metal complex , a silver n - heterocyclic carbene or mixtures thereof . the invention is also directed to a silver macrocyclic metal complex and a silver n - heterocyclic carbine , and methods of treating cancerous cells in a mammal including the steps of administering an effective amount of a silver metal salt , such as by incorporating the material into a biodegradable polymeric nanoparticle .