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.

Graphical abstract: 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

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Article information

Article type
Perspective
Submitted
04 Jun 2025
Accepted
23 Aug 2025
First published
03 Sep 2025

Dalton Trans., 2025, Advance Article

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

U. Das, A. K. Das, A. Das and D. Mondal, Dalton Trans., 2025, Advance Article , DOI: 10.1039/D5DT01316J

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