The fundamental nature of electron transfer between transition metal ion sites in electron-transfer proteins is being studied. Model complexes with electronic and structural characteristics similar to a particular metalloprotein metal site are being prepared and studied to understand what aspects of the electronic structure of the metal site are important in determining the biological functioning of the metalloprotein. Mixed-valence bridged ferrocenes, binuclear CuIICuI complexes, and iron porphyrins are being studied to determine the fundamental nature of electron transfer between metal ions and to detail the nature of the mixed-valence character of metal sites in iron-sulfur proteins. It is of interest to find out whether thermal electron transfer can occur between metal ions that are well separated or is electron tunneling involved. Binuclear metal complexes are being prepared to study magnetic exchange interactions between metal ions as propagated by extended bridging groups. Can bridges consisting of saturated groups propagate electron transfer between two metal ions that are well separated? Spin-crossover feric complexes are under study to elucidate the factors that control the spin-flipping rate in an individual spin-equilibrium molecule such as a cytochrome and the cooperativity of spin flipping seen for the solid state. Laser-flash photolysis experiments are being employed to measure the spin-state interconversion of isolated spin-crossover complexes. Direct evidence for tunneling is being obtained. Iron complexes with directly attached or remotely attached p-semi-quinone species are being prepared and characterized as models for the ubisemiquinone iron interaction detected in electron-transport chains.