Electron Transfer of Hydrated Transition-Metal Ions and the Electronic State of Co3+(aq)

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+, and Co2+/Co3+ using six distinct density functionals. We identify directly the ∼105 anomaly of Co2+/Co3+ from the electronic reorganization energies without the use of empirical cross-relations. Furthermore, when Co3+ is modeled as high-spin, the anomaly disappears, bringing all four processes on a linear trend within the uncertainty of the experiments and theory. We studied both the acid-independent [Co(H2O)6]3+ specie...