Thioamides have two tautomers, which form the same anion upon deprotonation (Figure , right). Most often, the thioamide coordinates as an iminothiolate through the S donor rather than the potential N donor as a result of the relative weakness of the C=S π bond and the better overlap of the S atom with metal orbitals. After coordination, protonation at the N atom gives a formally neutral thione donor that maintains the M-S interaction (Figure , brackets). Even as a formally neutral ligand, the thione C-S bond is significantly polarized toward a C δ+ -S δ-form that affords some anionic character at S. The ability to tune the donor properties of iron-coordinated sulfur via protonation/deprotonation of the supporting ligand could be used to probe how protonation state can affect the redox potential, spectroscopic characteristics, and reactivity of the iron center. The potential for proton responsiveness is also relevant to nitrogenase mechanisms. The Thorneley-Lowe kinetic scheme for nitrogenases proposes that one proton is transferred to the FeMoco with each reduction step, but the location of these protons has been controversial.