Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Roussin''s Red Salt is the inorganic compound with the formula [K2][Fe2S2(NO)4]. This metal nitrosyl consists of the potassium salt of the [Fe2N4O4S2]2 anion. This compound was first described in 1858 by the French chemist M.L. Roussin while experimenting with reactions between nitroprusside ion, [Fe(CN)5NO]2 , and sulfur, making it the first synthetic iron-sulfur cluster. Roussin''s red salt anion is an example of a ligand-bridged metal dimer. Its structure is described as 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.