Metal nitrosyls are complexes that contain transition metals bonded to nitric oxide, NO. Metal nitrosyls are biologically important. This large class of molecules is of interest to researchers and technologists.
Using the 18-electron rule, according to the neutral
electron counting NO can have two modes:
When the M-N-O angle is close to 180°, NO is a three-electron ligand (donates three electrons). This bonding mode, "linear nitrosyl", is common.
In some nitrosyl complexes, the M-N-O angle deviates strongly from linearity, approaching often 130°. In such "bent NO" complexes, NO is considered to be a one-electron, pseudohalide-like ligand. Such bent species are related to classical organic and main group nitrosyl compounds, for example nitrosobenzene and nitrosyl chloride.
Most transition metal nitrosyls are linear, most main group nitrosyls are bent.
Two of the earliest nitrosyl complexes are Roussin's Red Salt and Roussin's black salt. The Red Salt has the formula Na2[Fe2(NO)4S2]. The anion, [Fe2(NO)4S2]2−, can be viewed as an edge-shared bitetrahedron. Each Fe is bonded to two "linear" NO ligands and shares a pair of sulfido ligands with the other iron. The Black Salt is a more complex cluster with the formula Na[Fe4(NO)7S3]. The anion in this species has C3v point group symmetry. The Fe4S3 core is that of an incomplete cubane. The parent cubane is (FeNO)4S4.
Most metal nitrosyls are prepared by treatment of a metal with NO gas. Others have been prepared by oxidation of hydroxylamine complexes. A third method involves treatment of metal nitrito complexes with protic acids.
Since nitrogen is more electronegative than carbon, metal-nitrosyl complexes tend to be more electrophilic than related metal carbonyl complexes. Nucleophiles often add to the nitrogen.
The NO ligand also exhibits many reactions. The most important is the acid/base equilibrium:
This reaction illustrates the fact that linear nitrosyls can be viewed an acid anhydride.
In rare but noteworthy cases, NO is cleaved by metal centers:
The nitrogen atom in bent metal nitrosyls is basic, thus can be oxidized, alkylated, and protonated, e.g.: