# Structural and Electronic Complexities of a Sulfur‐Bridged Di‐Iron Complex Composed of Mono‐ and Di‐Nitrosyl Units

## TL;DR

This paper explores the stability and electronic behavior of a sulfur-bridged iron complex with nitrosyl units, revealing how electrons are distributed across its structure.

## Contribution

The study provides new insights into the electron distribution and structural changes in a sulfur-bridged di-iron complex with multiple redox states.

## Key findings

- Electron addition causes geometric and magnetic changes in the diiron complex.
- NO exchange occurs rapidly between mono- and dinitrosyl iron units during synthesis.
- Sulfur bridges and NO units effectively buffer and distribute electron density.

## Abstract

The delocalized, thermodynamically stable cation, [(N2S2)Fe(NO)•Fe(NO)2]+, an adduct of mono‐nitrosyl and dinitrosyl iron units, is analyzed to address the unusual stability of the sulfur‐bridged diiron complex in its three overall redox levels, +, 0, and −. X‐ray diffraction and myriad spectroscopic techniques probe products of sequential electron uptake in the corresponding neutral and anionic species. Conundrums include unified blueshifts of the overall 3‐band, ν(NO), pattern with added electrons. One‐electron reduction changes the anti‐ferromagnetically coupled, S = 0, cationic diiron species to the neutral analog, S = ½, with unpaired spin mainly localized on the MNIU, which decreases its ∠Fe–N–O angle by 10 degrees in response to the extra electron density. Subsequent reduction to the anionic species, S = 1, involves a major geometric change at the MNIU, which moves the Fe in {Fe(NO)}8 out of the N2S2 plane. Site‐specific 15N labeling of nitrosyl in the MNIU confirms the IR analysis and shows rapid NO exchange between the MNIU/DNIU (mono‐nitrosyl iron unit/dinitrosyl iron unit) pairs during its synthesis at RT. Mössbauer spectroscopy, S K‐edge XAS, and molecular orbital calculations confirm the ability of NO and the versatility of sulfur bridges to buffer and distribute electrons, a key to their major importance in metalloenzymes.

Two NO‐containing iron units, biologically relevant as NO storage/release centers, connected by sulfur, display characteristics arising from multiple quantum levels in their extensively delocalized arrangement. Analysis in three charge levels finds the electron distribution almost substantially spread over all atoms of its composition: 2S, 2Fe, and 3NO.