Electron transfer (ET) is broadly described by Marcus-type theories, and plays a central role in many materials and catalytic systems and in biomolecules such as cytochromes. Classic ET processes are the self-exchange reactions between hydrated transition metal ions such as Fe2+(aq) + Fe3+(aq) → Fe3+(aq) + Fe2+(aq). A well-known anomaly of Marcus theory is Co2+/Co3+ exchange, which proceeds ~105 times faster than predicted. Co3+(aq) is a complex and reactive system widely thought to feature low-spin Co3+. We studied the self-exchange process systematically for Cr2+/Cr3+, V2+/V3+, Fe2+/Fe3+, Co2+/Co3+ using six distinct density functionals. We identify directly the ~105 anomaly of Co2+/Co3+ from the electronic reorganization energies without use of empirical cross relations. Furthermore, when modeling Co3+ as high-spin, the anomaly disappears, bringing all four processes on a linear trend within the uncertainty of experiment and theory. We studied both the acid-independent [Co(H2O)6]3+ species that dominates at low pH, and the acid-dependent [Co(OH)(H2O)5]2+ species that becomes important at higher pH, and use two distinct explicit second-sphere hydration models and models of perchlorate anion association. The high-spin state with weaker Co-O bonds is stabilized by vibrational energy and entropy by ~11 and ~12 kJ/mol, correcting gap estimates from absorption spectroscopy. High-spin Co3+(aq) explains the full experimental data series of the M(aq). systems. Low-spin Co3+ and high-spin Co2+ involves changes in eg occupation upon electron transfer, with associated M-O bond changes and increased reorganization energy. In contrast, with high-spin Co3+(aq) the redox-active electrons shuffle between t2g orbitals to minimize structural changes, producing a relative rate in excellent agreement with experiment. This eg occupation effect explains most of the experimental differences in rate constants, with the remaining part explained by second-sphere hydration and anion effects. Our results consistently suggest that some high-spin Co3+(aq) is active during the experiments.
Nielsen, M. T., Moltved, K. A., & Kepp, K. P. (2018). Electron Transfer of Hydrated Transition Metal Ions and the Electronic State of Co3+(aq). Inorganic Chemistry, 57(13), 7914-7924. https://doi.org/10.1021/acs.inorgchem.8b01011