Nitric oxide complexes

As noted above, Priestley discovered the first transition metal complex of nitric oxide and in 1848 Lionel Playfair (1818-1898) reported red crystalline salts of [Fe(CN)5(NO)], 1 (commonly described as nitroprusside) using the following routes based on either concentrated nitric acid or nitrite salts  

Subsequently the corresponding vanadium, chromium, manganese, and cobalt salts [M(CN)5(N0)] anions were reported [3]. 

In solution sodium nitroprusside slowly releases cyanide ions, but not sufficiently rapidly, to prevent its historical use as an effective drug, which was administered intravenously, for malignant hypertension or for the rapid control of blood pressure during some surgical procedures [4—6]. The cyanide may be detoxified by thiosulphate ions, a process which is catalysed by the enzyme rhodanese. In the absence of sufficient thiosulphate, cyanide ions can quickly reach toxic levels. The half-life of nitroprusside is 1-2 min, but the metabolite 

Francois-Zacharie Roussin (1827-1894) discovered the first polynuclear cluster compound of nitric oxide as a result of his study of the reactions of iron(II) sulphate, ammonium sulphide and nitrous acid (HNO2), and this black crystalline material was subsequently characterised as a salt of [Fe4S3(NO)7] (Roussin s black salt), 2. It may be converted into a red salt on treatment with alkaline, which reacts with ethyl bromide to give Roussin s red ester, [Fe2(SEt)2(NO)4], 3 [7]. 

NO is also a strong Lewis acid and forms adducts with donors which do not oxidise readily, e.g. Me2SO. Formally the nitrosyl halides shown in 6 may be viewed as NO adducts of the halide anions X. The independent chemistry of reduced nitric oxide (NO ) is limited, and it probably has an extremely short independent life in biological media [1, 6]. NO is a Lewis base, which is isoelectronic with O2 and formally forms adducts with Lewis acids such as giving angular HNO. 

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Other molybdenum(II) compounds of interest include the nitric oxide complexes Mo(NO)Cl3 and Mo(NO)(dtc)3, /n j -MoH2(dppe)2 where dppe = l,2-bis(diphenylphosphino)ethane and K4 Mo2(SO J -2H20. 

Metal carbonyls form several complexes but those of major interest in MOCVD are the carbonyl halides and the carbonyl-nitric-oxide complexes. 

In comparison to the spectra of the nitric oxide complexes, the optical spectra of the ferric and ferrous forms of NPl-4 in the absence of NO over the pH range of 5.5 to 7.5 show the more typically expected changes in Soretband maximum reported previously for NPl (49). The high-spin NP species have their Soret band maxima at 402-403 nm, whereas the high-spin NP have their Soret band maxima at 430 nm at pH 5.5 for NPl and NP4 and 417-418 nm for NP2 and NP3. In all cases the a, 3 bands are quite broad and of low extinction coefficient (50). 

Table 10 Kinetic parameters and equilibrium constants for formation and dissociation of ternary nitric oxide complexes (in water, at 298 K). 

Reaction of the manganese tropocoronand complex [Mn(tc-5,5)(NO)] with [Fe(tc-5,5)] results in complete transfer of the NO to the [Fe(tc-5,5)]. Other nitric oxide complexes appear in the sections on nitroprusside (Section S.4.2.2.6 above), on phthalocyanines (Section 5.4.3.7.4 above), and on polynuclear iron-sulfide complexes (Roussin s salts Section 5.4.5.9.2 below) Fe-por-phyrin-NO redox chemistry has been mentioned in Section 5.4.3.7.2 above. 

To separate osmium from ruthenium, the aqueous solution is acidified with nitric acid. While nitric acid oxidizes osmate ion to volatile osmium tetroxide, Os04, it converts ruthenium to a nitric oxide complex. Osmium tetroxide is removed from the solution by distillation in air and collected in an aqueous solution of caustic soda containing ethanol. Osmium tetroxide solution is heated with ammonium chloride, upon which osmium precipitates out as a complex chloride, 0s02(NH3)4Cl2. The precipitate is filtered, washed and decomposed by ignition with hydrogen to yield osmium metal. 

Nitric Oxide Complexes of Metalloproteins An Introductory Overview... 

V. NITRIC OXIDE COMPLEXES OF IRON-SULFUR PROTEINS... 

Reddy, D., Lancaster, J. R., Jr., and Comforth, D. P. (1983). Nitrite inhibition of Clostridium botulinum Electron spin resonance detection of iron-nitric oxide complexes. Science 221, 769-770. 

Woolum, J. C., Tiezzi, E., and Commoner, B. (1968). Electron spin resonance of iron-nitric oxide complexes with amino acids, peptides and proteins. Biochim. Biophys. Acta 160, 311-320. 

J. Lewis, Transition Metal—Nitric Oxide Complexes , Sci. Prog. (Oxford), 1959, 47, 506. 

Nitrosylmyoglobin (NO-Mb) Nitric oxide complex with myoglobin Intact Fe(II) Native Red... 

At the present time we have no certain knowledge of the state of the heme in these 450 nm species. We do not know if there are heme aggregates although they are unlikely. It is therefore reasonable to look at systems where the haem is aggregated as well as those where it is not in order to see how the absorption spectra can be mimic-ed. It seems reasonable to assume that the iron is low-spin in the carbon monoxide, isocyanide, and nitric oxide complexes as no high-spin iron complexes of this type are known. In the high-spin or low-spin state it may be that the thiol is weakly bound, if at all, for Fe(II) heme in models or in hemoglobin does not bind to thiols. In an attempt to understand these spectra we shall use a semi-empirical approach based on the theoretical discussion in the previous article (52) and elaborated in what follows immediately. Only Fe(II) complexes will be analysed as the Fe(III) proteins have been previously examined (52). 

These results have been discussed by Seel in Structure and Valence Theory of Inorganic Nitric Oxide Complexes (102). In such complexes, nitric oxide is covalently bonded to the metal atom as the positive ion NO+, it being assumed that an electron is transferred to the metal. This allows the isoelectronic groups NO+, CO, CN to be considered together and permits understanding of such known series as [Fen(CN)5NO]2, [Fen(CN)5CO]3-, and [Fen(CN)5CN]4-. 

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