Transition metal nitroso complexes

Transition metal nitroso complexes are coordination complexes containing one or more organonitroso ligands (RNO).^[1]

Structure of the dye Naphthol Green B, which features of nitroso ligand bound to Fe(III).

Not to be confused with Metal nitrosyl complex.

Structure and bonding

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. Illustrative are Ru(acac)₂(C₆H₅NO)₂, where a pair nitrosobenzenes are monodentate, and [Ru(acac)₂(μ−C₆H₅NO)]₂ where two nitrosobenzenes bridge.^[2]

Chelating nitroso ligands

Trapping of 1,2-dinitrosobenzene by Ru(II)

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.^[3]

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).^[4] 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.^[5]

Ni(PEt₃)₄ + i−PrNO₂ → Ni(PEt₃)₂(η²-i−PrNO) + PEt₃ + OPEt₃ (Et = C₂H₅, i-Pr = (CH₃)₂CH)

The coupling of organic ligands and nitric oxide is yet another route.^[1]

Connection to methemoglobinemia

Structure of Fe(octaethylporphyrin)(C₆H₅NO)(imidazole). This synthetic complex is thought to resemble heme inhibited by nitroso benzene. Color code: red = O, blue = N, Fe, gray = C, white = H.

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.^[6]

Related compounds

As indicated by the applications in dyeing, chelating aryl nitroso compounds often form deeply colored complexes

• Cupferron, C6H5N(O-)NO, an anionic O-O chelating proligand was once a popular reagent for the analysis of metal ions.
• Millon's reagent, which involves nitroso-based ligands, was once a test for proteins