Of coordinated nitrosyl

The bis-hydroxylamine adduct [Fe (tpp)(NH20H)2] is stable at low temperatures, but decomposes to [Fe(tpp)(NO)] at room temperature. [Fe(porphyrin)(NO)] complexes can undergo one-and two-electron reduction the nature of the one-electron reduction product has been established by visible and resonance Raman spectroscopy. Reduction of [Fe(porphyrin)(NO)] complexes in the presence of phenols provides model systems for nitrite reductase conversion of coordinated nitrosyl to ammonia (assimilatory nitrite reduction), while further relevant information is available from the chemistry of [Fe (porphyrin)(N03)]. Iron porphyrin complexes with up to eight nitro substituents have been prepared and shown to catalyze oxidation of hydrocarbons by hydrogen peroxide and the hydroxylation of alkoxybenzenes. ... 

General accounts of the reactivity of coordinated nitrosyls are available.106 121 124 A metal ion or metal complex is unable to generate the reactivity of ionic NO+ towards nucleophiles (i.e. OH-, RS-, RHN-) or of NO- towards electrophiles (i.e. H+, PhCH2X), but it can present NO in a less reactive form under conditions inappropriate to the free ion. Thus linear coordination with vNO > 1886 cm 1 may promote nucleophilic reactions at N, and bent coordination with the higher electron density on N (vNO < 1700cm-1) may promote electrophilic attack by H+, 02, NO and PhCHjBr.125 No cobalt nitrosyl is known to undergo nucleophilic attack on the nitrosyl group. Normally linear coordination is associated with four or five coordination and the nucleophile adds to the unsaturated metal instead. However a number of electrophilic reactions are known (Table 13). A recent discussion of these is recommended.124... 

An important reaction of NO gas, catalyzed by transition metal surfaces, is reduction with hydrogen to give ammonia. As a consequence, a great deal of effort has been directed into studying the reduction of coordinated nitrosyl ligands. 

A series of electrophilic reactions of coordinated nitrosyl group has been reported. [RuCl-(NO)(bipy)2] " reacts with aryl amines ArNHj to give [RuCl(N2Ar)(bipy)] which shows a degree of diazonium ion character with range 1980-2100cm . " " Cleavage of these... 

In 1895, Hofmann studied the reduction of coordinated nitrosyl groups. The interaction of sodium nitroprusside with hydrazine in an aqueous solution of NaOH formed the corresponding aqua-complex with the explosive liberation of dinitrogen. The overall process is described by Eq. 3.10 ... 

Many of the nitrosyls studied are 5-coordinate, and analysis of crystallographic results indicates that, in general, in the trigonal bipyramid structures NO is found in the equatorial position in a linear geometry whereas in a square pyramidal structure, there is a bent M—N—O linkage in an apical position. A further point of interest is that in compounds like Ir(NO)Cl2(PPh3)2, the nitrosyl group bends in the more hindered (P—Ir—P) plane. 

Electrophilic reactivity and structure of the coordinated nitrosyl group. J. Masek, Inorg. Chim. Acta, Rev., 1969,3, 99-108 (36). 

The low reactivity of both Cyt111 and Cyt11 toward NO can be attributed to occupation of the heme iron axial coordination sites by an imidazole nitrogen and by a methionine sulfur of the protein (28). Thus, unlike other heme proteins where one axial site is empty or occupied by H20, formation of the nitrosyl complex not only involves ligand displacement but also significant protein conformational changes which inhibit the reaction with NO. However, the protein does not always inhibit reactivity given that Cat and nNOS are more reactive toward NO than is the model complex Fem(TPPS)(H20)2 (Table II). Conversely, the koS values... 

Some other reactions of metal nitrosyls LxM(NO) with various nucleophiles (Nuc) are summarized in Table III. The pattern indicated by the studies described above is repeated simple adduct formation occurs when the coordinated nitrosyls are sufficiently electrophilic and the nucleophiles sufficiently basic. The first species formed is probably the N-coordinated nucleophile nitrosyl adduct LrM(N(O)Nuc), e.g. Eq. (27). Subsequent reactions depend on the substitution lability of these species, as well as on the redox stability of the complex and of the ligand. 

A number of compounds containing nitroso or coordinated nitrosyl groups exhibit instability under appropriate conditions. Individually indexed compounds are Ammonium /V-nitrosophcnvlaminooxidc. 2399 A-(2-Chlorocthyl)-A-nitrosocarbamoyl azide, 1131 1 -Chloro-1 -nitrosocyclohexane, 2419 2-Chloro-1 -nitroso-2-phenylpropane, 3147... 

B41. P. S. Braterman, ed. Reactions of Coordinated Ligands Vol. 1, Plenum 1986. Comprehensive but mainly descriptive Vol. 2, Plenum, 1989 covers coordinated COj, Nj, nitrosyls, O- and N-bound,... 

Ru(CN)jNO reactions with OH , SH and SOj" resemble those of the nitroprusside ion, with attack at the coordinated nitrosyl to give analogous transients and similar second-order rate constants. Ruthenium(II) complexes of the general type Ru(N2), Nj = biden-tate hgands, are important reactants. The relative inertness of Ru(NH3) + and Ru(diimine)f+ towards substitution makes these complexes definite, although weak, outer-sphere reductants (Tables 5.4, 5.5, 5.6 and 5.1). Ruthenium(ll) complexes of the general type Ru(diimine)f +, and particularly the complex Ru(bpy)j+, have unique excited state properties. They can be used as photosensitizers in the photochemical conversion of solar energy. Scheme 8.1 ... 

Other useful correlations exists between SEj, and the ability of the metal to backbond to the nitrosyl as reflected in the Tc-NX (X = O, S) bond distance (see Fig. 5 [96] and the NO stretching frequency in Ru" amine complexes (Fig. 6). As EEl increases, the ability of the metal to backbond decreases and the coordinated NO becomes more like NO". Such correlations should prove useful not only for predicting the ability of metallonitrosyls to release NO, but also the tendency of the nitrosyl to undergo nucleophilic attack. 

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