Co(III) complexes are known as inert (characterized by slow ligand exchange) but after reduction, the Co(II) state that is formed is labile (fast ligand exchange). Hypoxia as a common feature of cancer tissues means lack of adequate oxygen level, therefore a more reductive environment. When administered, rationally designed Co(III) complexes may be reduced selectively in these tissues and therefore may serve as carrier of drug molecules that are capable of killing cancer cells because the reduction and the release of the drug is more pronounced in the cancer cells. The type of drug molecules can be hydroxamic acids with proven anticancer activity.