Differential Impacts of Pyrophosphate on Ferrates(VI, V, and IV): Through Its Unique Inhibition to Identify Fe(V) Species

High-valent iron species [Fe­(V) and Fe­(IV)] exhibit remarkable oxidative activity in environmental chemistry. However, the distinctions between the properties of Fe­(V) and Fe­(IV) remain poorly understood due to the challenges of distinguishing them. Herein, using pyrophosphate as a model ligand, we comprehensively investigated the influence of oxo-ligands on the reactivity of high-valent iron­(VI, V, IV) species. An innovative strategy to selectively generate Fe­(IV) using the Fe­(VI)-initiated system was proposed, enabling an in-depth investigation of the interaction between Fe­(IV) and pyrophosphate. The results reveal that pyrophosphate strongly inhibits Fe­(V) oxidation, while it has minimal impact on the reactivity of Fe­(VI) and Fe­(IV). Based on ligand field theory, pyrophosphate complexation can induce iron 3d orbital resplitting, leading to spin electron rearrangement. Specifically, the hexa-coordinated Fe­(V)-oxo complex ligated by pyrophosphate exhibits higher orbital energy, reducing its stability and effective collisions with contaminants, whereas, the potential Jahn–Teller distortion of the Fe­(IV)-oxo complex could enhance its stability and preserve its significant reactivity. Given its selective inhibition of Fe­(V) oxidation, pyrophosphate can emerge as a promising targeted quenching agent for Fe­(V) species. This study provides valuable theoretical insights to guide the identification and characterization of intermediate iron species in iron-based oxidation processes.