Present state of knowledge on the electronic structure and oxidation state of iron in the brown ring complex: a long-standing debatable issue in undergraduate chemistry courses†
Abstract
In terms of electronic structure, the century-old, well-known brown ring complex formed in the classical brown ring test employed in the qualitative analysis of nitrate (and nitrite) is still a matter of debate among both theoretical and experimental chemists. The extensive π-bonding in the Fe–N–O linkage complicates the prediction of the real oxidation state of iron in the brown ring complex, involving the non-innocent NO ligand that can have different chemical states to tune the oxidation state of iron in the complex. This article summarises the present-day knowledge of the possible electronic structures of the quartet {Fe(NO)}7 moiety (S = 3/2) of the brown ring complex. Experimental observations (isomer shift of 57Fe in MB spectrum, EPR and IR data, X-ray crystal structure data, etc.) along with the consideration of the basic required conditions to accommodate the large number of remaining σ-donor neutral ligands (5H2O) support the most probable oxidation state of iron as an intermediate state of +2 and +3, but closer to +3.
- This article is part of the themed collection: 2025 Frontier and Perspective articles