Nitrosyl anion

Furthermore, Laranjinha and Cadenas [58] have recently showed that nitric oxide oxidizes 3,4-dihydroxyphenylacetic acid (DOPAC) to form nitrosyl anion and the DOPAC semiqui-none supposedly by one-electron transfer mechanism. 

The use of appropriate tetradentate N202-donating Schiff base ligands (Fig. 14) together with the incorporation of the N-donating nitrosyl anion has resulted in the formation of a unique series of five-coordinate Fe(III) spin crossover materials containing FeN302 chromophores. 

Br2 or HN03 oxidizes the nitrosyl anion to [Mn(CN)5NO]2-, and the standard reduction potential56 for this reversible process is 0.6 V. Methylation62 with methyl fluorosulfonate gives [Mn(CNMe)5N0](S03F)2. 

Since N02 is best suited for attacking neutral clusters and is best at nitrosylating anionic clusters, the successive combination of these two reagents offers a feasible method toward polynitrosyl clusters. To date this has been used only once [Eqs. (23) and (24)] (34). It is conceivable (although admittedly unlikely) that such a one-two punch could be... 

Several iron sulfide nitrosyl compounds are known. These have stmctures that in some cases are formally related to the FeS clusters by replacement of thiolate by NO. The compounds include the anions [Fe2S2(NO)4] and [Fe4S2(NO)2] (Roussin s red and black salts, respectively) and the neutral compounds [Fe2S2(NO)4] and [Fe4S4(NO)4]. Roussin s black salt has found use as a NO releasing vasodilator. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

For ruthenium, electrolytes based on ruthenium sulphamate or nitrosyl-sulphamate have been described, but the most useful solutions currently available are based on the anionic complex (H2 0 Cl4 Ru N Ru-Cl4-OH2) . The latter solutions operate with relatively high cathode efficiency to furnish bright deposits up to a thickness of about 0-005 0 mm, which are similar in physical characteristics to electrodeposited rhodium and have shown promise in applications for which the latter more costly metal is commonly employed. Particularly interesting is the potential application of ruthenium as an alternative to gold or rhodium plating on the contact members of sealed-reed relay switches. 

Ruthenium probably forms more nitrosyl complexes [115] than any other metal. Many are octahedral Ru(NO)Xs systems, where X5 can represent a combination of neutral and anionic ligands these contain a linear (or very nearly) Ru-NO grouping and are regarded as complexes of ruthenium(II). They are often referred to as (Ru(NO) 6 systems. 

The low-valent ferrate [Fe(CO)3(NO)] 76 or Hieber anion was discovered some 50 years ago by Hieber and Beutner [43, 44] in order to extend the Hieber base reaction [45,46], in which iron pentacarbonyl 78 reacts with alkaline bases to form the [Fe(CO)4] anion [47, 48]. Compared to its homoleptic analogue, the Hieber anion is more stable because the electron-withdrawing character of the nitrosyl ligand stabilizes the negative charge at the iron atom. 

Fig. 4. Illustration of limiting cases of NO binding to a metalloporphyrin center as (a) the nitroxyl anion (NO-) with a M-N-0 bond angle of 120° or as (b) the nitrosyl cation (NO+) with a M-N-0 bond angle of 180°.

Studies in this laboratory (69) of the water soluble ferri-heme model Fem(TPPS) in aqueous solution have shown that this species also undergoes reductive nitrosylation in solutions that are moderately acidic (pH 4-6) (Eq. (32)). The rate of this reaction includes a buffer dependent term indicating that the reaction of the Fem(TPPS)(NO) complex with H20 is subject to general base catalysis. The reaction depicted in Eq. (33) is not observable at pH values < 3, since the half-cell reduction potential for the nitrite anion (Eq. (1)) is pH dependent, and Eq. (33) is no longer thermodynamically favorable. 

The syntheses, structures and properties of wide varieties of metal nitrosyl complexes have been well documented [4, 5, 20-23]. However, the bulk of the complexes reviewed previously are of academic interest and only a few of these metal nitrosyl complexes have been considered as biologically effective NO donors. It was observed that the metal nitrosyls with significant NO+ character are subject to attack from a variety of nucleophiles and have hypertensive properties. This could be due to the strong trans- labilizing effect of NO. In contrast, the metal nitrosyl compounds with the general formula [M(CN)5NO]n, where the NO ligand was either neutral (for M = Co) or anionic (for M = Cr) showed no vasodilatory effect [24]. 

Amazingly, there is no systematic cis effect of the ligand L trans to carbon monoxide in the carbonylosmium(II) porphyrins Os(OEP)CO(L) [30a-30g], (Table 10). In the nitrosyl derivatives, Os(OEP)NO(L), the anionic ligand Le exerts only a small effect (Table 11) the wavelengths of the a-bands increase in the series [31c] < [3Id] <... 

As a result of their very low reactivity with the HO anion, the organic nitrates show a practically pH-independent (and very slow) hydrolysis at pH 7 and above [27], In contrast, they will react rapidly with alcohols in nonpolar media by nitrosyl exchange (Fig. 9.2,b) [28]. Formally, H+ and NO+ (nitrosyl cation) are exchanged, but the actual reaction mechanism is quite complex. The reaction leads to an equilibrium, which depends on the partners and presumably also on conditions. However, the biological relevance of this reaction remains to be understood. 

Silyl radicals have been produced by one-electron oxidation of silyl metals [11]. This is found to be the method of choice for the generation of persistent silyl radicals and allowed the preparation of the first isolable silyl radical (see later in this chapter). Reactions (1.5) and (1.6) show two sterically hindered silyl anions with Na+ as the counter-cation, and their oxidation by the nitrosyl cation [12] and the complex GeCh/dioxane [13], respectively. 

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