Roussin's red salt

Roussin's red salt is the inorganic compound with the formula K₂[Fe₂S₂(NO)₄]. This metal nitrosyl was first described by Zacharie Roussin in 1858, making it one of the first synthetic iron-sulfur clusters.^[1][2]

Roussin's red salt

Names
IUPAC name potassium tetranitrosyl-di-μ-sulfidodiiron(Fe–Fe)(2–)
Other names Ferrate(2-), tetranitrosyldi-mu-thioxodi-, (Fe-Fe), dipotassium
Identifiers
CAS Number	58204-17-4
3D model (JSmol)	Interactive image
ChemSpider	142967
PubChem CID	162850 (formula error)
CompTox Dashboard (EPA)	DTXSID20973767
InChI InChI=1S/2Fe.4NO.2S/c;;4*1-2;;/q;;4*+1;2*-2 Key: HAJABACXLZMFIH-UHFFFAOYSA-N
SMILES [K+].[K+].O=N[Fe-]12(N=O)S[Fe-]1(S2)(N=O)N=O
Properties
Chemical formula	Fe2N4K2O4S2
Molar mass	374.04 g/mol
Appearance	Dark red crystals
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Structure and bonding

Roussin's red salt anion is an edge-shared bitetrahedron, wherein a pair Fe(NO)₂ units are bridged by a pair of sulfide ligands. The Fe-NO bonds are linear indicating NO is acting as a three electron donor.^[3] The diamagnetic compound obeys the 18-electron rule. The dark red colour of the complex is attributed to a number of charge-transfer interactions between the iron core and nitrosyl ligands.^[4]

Synthesis

Roussin's red salt solution.

The French chemist Z. Roussin^[5] first prepared this salt while investigating reactions between nitroprusside ion ([Fe(CN)₅NO]²⁻) and sulfur.^[6] The salt can be prepared by the reaction of sulfide salts with iron nitrosyl halides:^[7]

Fe₂I₂(NO)₄ + 2 Li₂S → Li₂Fe₂S₂(NO)₄ + 2 LiI

Another way to obtain Roussin's red salt is to alkalize a solution of the related compound Roussin's black salt, K[Fe₄S₃(NO)₇], using a suitable base:

2 K[Fe₄S₃(NO)₇] + 4 KOH → 3 K₂[Fe₂S₂(NO)₄] + 2 Fe(OH)₂ + 2 NO

Another obtaining procedure consists of reacting sodium (or potassium) nitrite, ascorbic acid, ferrous sulfate, sodium sulfide and sodium (or potassium) hydroxide in this order. Ascorbic acid works both as an acidulant and as a reducing agent, converting nitrite into nitrous acid and then reducing it to nitrogen monoxide, which will then bind to iron ions forming nitrosyl complexes, which will react with sulfide in a basic medium giving Roussin's red salt.^[8]

2 FeSO₄ + 4 HNO₂ + 3 C₆H₈O₆ + 2 Na₂S + 2 NaOH → Na₂[Fe₂S₂(NO)₄] + 3 C₆H₆O₆ + 2 Na₂SO₄ + 6 H₂O

To obtain the "esters", the salt is alkylated:

Li₂Fe₂S₂(NO)₄ + 2 RX → Fe₂(SR)₂(NO)₄ + 2 LiX

Esters can also be easily be prepared from the reaction of Fe₂I₂(NO)₄ with the thiol or thiosulfate.

Occurrence and potential applications

It is found in nature as its "esters" with the formula Fe₂(SR)₂(NO)₄, where "R" is any alkyl group.^[3] In addition Roussin's red salt is discussed in the fields of microbiology and food science due to its mutagenic properties.^[9]

The ester derivative are being investigated as nitric oxide donors in biology and medicine, due to the relatively low toxicity and good stability of Roussin's red salt.^[10]Photodissociation of the compound induces the release of NO, thereby sensitizing target cells to exposure to radiation.^[9]

See also

• Roussin's black salt