Transition metal nitroso complexes are coordination complexes containing one or more organonitroso ligands (RNO).
Organic nitroso compounds bind to metals in several ways, but most commonly as monodentate N-bonded ligands. Also known are O-bonded, ÷<sup>2</sup>-N,O-bonded. Dimers of organic nitroso compounds also bind in a ú<sup>2</sup>--O,O bidentate manner. Illustrative are , where a pair nitrosobenzenes are monodentate, and where two nitrosobenzenes bridge.
Arylnitroso compounds with a flanking hydroxy group are a well-developed, e.g. 1-nitroso-2-naphthol. They are precursors to anionic N,O chelating ligands. Chelating dinitrosoarenes are uncommon but have been investigated.
Organic nitroso complexes can be prepared from preformed organic nitroso precursors. These precursors usually exist as N-N bonded dimers, but the dimer dissociates readily. This direct method is used to give W(CO)<sub>5</sub>(tert-BuNO) (where tert-Bu is ). The Fe-porphyrin complex depicted below is prepared by this route. More complicated but more biorelevant routes involve degradation of precursors such as nitrobenzene and phenylhydroxylamine.
The coupling of organic ligands and nitric oxide is yet another route.
Methemoglobinemia is a disorder where a large fraction of hemoglobin in one's blood has converted to inactive forms, generically called methemoglobin. Since methemoglobin is not an oxygen-carrier, methemoglobinemia is a serious disorder, sometimes fatal. Exposure to nitrobenzene, aniline, and their derivatives cause this disorder, which is attributed to their conversion to nitrosobenzene (and derivatives), which inactivate hemoglobin by forming a complex with the Fe center, precluding binding of O<sub>2</sub>.
As indicated by the applications in dyeing, chelating aryl nitroso compounds often form deeply colored complexes