During redox reactions, electrons hop between molecules. Dr. Taube demonstrated that electron transfer between metal complexes proceeds via two distinct pathways, called outer-sphere and inner-sphere mechanisms, and he established how the reaction rate depends on each route. Using isotopic labeling and fast spectroscopy, he quantified how a bridging ligand speeds or slows the process and formulated the reaction mechanism. These insights are fundamental for designing fuel-cell catalysts and for understanding many enzyme reactions in living organisms.

Taube focused on the redox potentials, primary coordination spheres, and bonding modes of transition-metal complexes and classified electron-transfer reactions on kinetic and mechanistic grounds. In outer-sphere transfer the complexes keep their coordination shells intact, receive a reorganization energy, and pass through an activation barrier described by Marcus theory. In inner-sphere transfer, a covalent bridging ligand forms transiently, and σ/π overlap in end-on or side-on bonding governs electron flow. By labeling ligands with isotopes such as ^18O and ^36Cl, Taube traced ligand migration and confirmed the presence of reaction intermediates. His work bridges coordination chemistry, electrochemistry, and bioinorganic chemistry, finding application in photosystem II studies and cytochrome electron chains.

Working primarily with Ru(III)/Ru(II) and Co(III)/Co(II) couples, Taube quantified electron-transfer rate constants spanning 10^−9 to 10^3 M^−1 s^−1 and introduced coordination-sphere reorganization energy and bridge-formation energy as explicit reaction-coordinate terms. For outer-sphere pathways he matched experimentally derived λ and ΔG‡ values to Marcus theory, while for inner-sphere routes he trapped μ-Cl and μ-OH one-electron shared intermediates by low-temperature EPR and pulse radiolysis. Isotope effects and solvent free-energy correlations further clarified conditions under which proton-coupled electron transfer (PCET) becomes operative. These achievements provide a theoretical scaffold for multi-electron molecular catalysis, artificial photosynthetic devices, and pathway analysis in membrane-bound metalloproteins.