As the role of transition metal ions in biological systems becomes better understood, it has become apparent that various biological ligands strongly modulate the oxidation-reduction behavior of metal ions. Less well studied but no less important are the effects exerted by transition metal ions on the redox behavior of biologically important molecules, particularly those which normally function as electron transfer agents. Striking examples of this have been shown for a series of MgIIand ZnIIporphyrin complexes in which substitution of the metal ion for two protons sufficiently lowers the oxidation potential of the porphyrin so that it becomes fairly oxidizable in two successive 1esteps.1Sirohemes in nitrite and sulfite reductases catalyze a remarkable 6ereduction in which the reducing ability of both the iron and the isobacteriochlorin are important.2,3Metal ions are particularly important in modifying the redox behavior of substrates which coordinate in the active sites of metalloenzymes. Examples of this are nitrogenase and the series of enzymes containing the xanthine-oxidase type of molybdenum center.4Some copper enzymes, such as ascorbate oxidase5also appear to coordinate the substrate directly in order to effect an efficient electron transfer. Oxygen coordination to copper appears to be especially useful in facilitating critical reductions of O26