The present invention relates to purified platinum complexes in which platinum is coordinated to form a single N,O- or O,O-chelate of an amidomalonate. The chelates, and pharmaceutically acceptable formulations thereof, are useful for the treatment of cancer. The platinum chelates may additionally be linked to one or more functional groups which increases water solubility and/or assists in tumor targeting of the chelate. Examples of tumor targeting moieties include polymers, in which one or more platinum chelates are attached to the polymer backbone via linkers capable of being cleaved in the body, and molecules with a high affinity for receptors which are concentrated or upregulated in tumor tissue and/or the tumor vasculature.