or change in valence (from Fe 2+ to Fe 3+ to Fe 4+ ). When assessing the spectra with regards to the two suggested reaction paths, a Fe 4+ signal should be accompanied by a Fe 2+ signal for route A. The spectra clearly show no sign of Fe 2+ . Therefore, the emerging peak is likely not a pure HS Fe 4+ -C species but rather a variety of the HS Fe 3+ -C signal that has a different coordination environment to BG in accordance with route B. It may also be a partially oxidised species, often denoted Fe 3+δ+ depending on the nature of the added ligands.