One of the earliest examples of a nitrosyl complex to be synthesized is Roussins red salt, which is a sodium salt of the anion [Fe(NO)S]. The structure of the anion can be viewed as consisting of two tetrahedra sharing an edge. Each iron atom is bonded linearly to two NO ligands and shares two bridging sulfidi ligands with the other iron atom. Roussins black salt has a more complex cluster structure. The anion in this species has the formula [Fe(NO)S]. It has C symmetry. It consists of a tetrahedron of iron atoms with sulfide ions on three faces of the tetrahedron. Three iron atoms are bonded to two nitrosyl groups. The iron atom on the threefold symmetry axis has a single nitrosyl group which also lies on that axis.