The overall objective of the proposed project is to evaluate transfer reactivity of intermediate and high spin iron porphyrins. Both mononuclear and binuclear ferrous and ferric complexes will be studied. It is proposed to use primarily electrochemical methodologies to characterize, in non-aqueous media, rates and thermodynamics of electron transfer as a function of iron-porphyrin plane distance, spin state of the metal, and porphyrin ring basicity. Thermodynamics of ligand addition to iron(II) and iron(III), as well as entropies of electron transfer will be calculated. The chemical and electrochemical reactivity of several mu-oxo and mu-nitrido bridged dimers containing Fe(IV), (III), (II), and (I) will also be investigated with an aim toward obtaining extremes in standard rate constants and stability of unusual oxidation state complexes. Mechanisms and rates of electron transfer, as well as thermodynamics of ligand binding, will be elucidated as a function of porphyrin ring substituents. By synthesis of porphyrins containing judiciously placed electron-donating or electron-withdrawing substituents, we will attempt to obtain model compounds which exhibit either extremely sluggish redox kinetics or especially rapid electron transfer rates.