Impact of the Metal–Ligand Bonding Nature on Electronic States of {Fe(NO)2}9/10–{Fe(NO)2}9/10 RREs Unraveled by the Combined X-ray Spectroscopic Analyses: Relevance to DNIC-Mediated Translational Functionality

Probed by the combined X-ray spectroscopic techniques (N K-edge/Fe L-edge XAS, Fe K-edge HERFD–XANES, Kβ1,3 mainline, VtC Kβ2,5 XES) and DFT computation, the qualitative/quantitative assignment of Fe/NO oxidation states in dinuclear dinitrosyl iron complexes (DNICs) suggests that the balance between NO– → Fe π-donating and Fe → •NO π-backbonding interactions in the [Fe(NO)2] unit(s) is modulated by the ionicity-covalency competition within the Fe–ER(2) bonds for a series of [ER(2)]−-bridged {Fe(NO)2}9/10–{Fe(NO)2}9/10 RREs (E = S, N, O). With the delicately ligand-controlled NO–/•NO equilibrium in the [Fe(NO)2] unit, the biomimetic activity governed by {Fe(NO)2}9/{Fe(NO)2}10 DNICs echoes their electronic/spectroscopic features, which not only uncovers molecular identity in complex environments but also decodes the reported H2S/NO communication, degradation/repair of [Fe–S] clusters, and translational functionality for NO-to-[N2O2]2–-to-N2O and NO-to-HNO/NO– transformations.