Transition metal nitroso complexes

Transition metal nitroso complexes are coordination complexes containing one or more organonitroso ligands (RNO).

Structure and bonding

 * [[image:NaphtholGreenB.svg|thumb|right|Structure of the dye [[Naphthol Green B]], which features of nitroso ligand bound to Fe(III).]]

Organic nitroso compounds bind to metals in several ways, but most commonly as monodentate N-bonded ligands. Also known are O-bonded, η2-N,O-bonded. Dimers of organic nitroso compounds also bind in a κ2--O,O bidentate manner.

Synthesis
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)5(tert-BuNO) (where tert-Bu is (CH3)3C). 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.
 * Ni(PEt3)4 + i\sPrNO2  ->  Ni(PEt3)2(\h{2}i\sPrNO)  +  PEt3  +  OPEt3  (Et = C2H5, i-Pr = (CH3)2CH)

The coupling of organic ligands and nitric oxide is yet another route.

Connection to methemoglobinemia
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 O2.