Nitrosyl complexes chemistry

The coordination chemistry of NO is often compared to that of CO but, whereas carbonyls are frequently prepared by reactions involving CO at high pressures and temperatures, this route is less viable for nitrosyls because of the thermodynamic instability of NO and its propensity to disproportionate or decompose under such conditions (p. 446). Nitrosyl complexes can sometimes be made by transformations involving pre-existing NO complexes, e.g. by ligand replacement, oxidative addition, reductive elimination or condensation reactions (reductive, thermal or photolytic). Typical examples are ... 

In summary, a complex chemistry could be envisaged for explaining the formation of N2 and N20 under lean conditions. In the particular case of N20, nitrosyl, dinitrosyl species as well as nitrates and nitrites species could be considered as intermediates particularly under lean conditions. 

Thionitrosyl compounds are less common, but the few structurally characterized representatives suggest a close similarity to the chemistry of nitrosyl complexes. 

Nitrosyl complexes, in which Vno > 1886 cm or Fnq > 13.8 mdyn/A, usually react as electrophilic nitrosating agents so that the ligand can be considered NO [26]. Nucleophilic attack on the nitrosyl nitrogen is a common reaction encountered in the chemistry of nitroprusside and the rates and activation parameters for a number of different nucleophiles are listed in Table 3. Hydr-oxylamine adducts to nitroprusside via a rate law that is first order in the complex, the ligand and hydroxide (k = 4.5 x 10 M s ). 

Because this chapter includes material that may be difficult for interested workers with different backgrounds, an attempt has been made to organize sections so that some material may be skipped without loss of continuity. The boxed sections provide some insight into aspects of the physical chemistry of metal nitrosyl complexes, but the chapter can be read without them. 

Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25).

The chemistry of a number of carbonyl and cyclopentadienylchromium-nitrosyl complexes has been discussed in the companion series.1... 

The recent upsurge of interest in iron-sulfur-nitrosyl complexes has been stimulated in part by the reported isolation of [Fe2(SMe)2(NO)4] from natural sources (12), by the obvious resemblances between these complexes and the naturally occurring [2Fe-2S] and [4Fe-4S] clusters of iron sulfur proteins (23, 14), and by the connections between tetrairon-sulfur-nitrosyls and cubane-type clusters (15). Most of the work in this area has been published in the past 5 years or so, and no review has previously been made. However, a number of excellent reviews of the wider aspects of metal-nitrosyl chemistry have appeared (16 19). 

The central role of the complexes [Fe(NO)2(SR)2]- in the reaction chemistry of iron-sulfur-nitrosyl complexes and their very ready formation both in in vitro and in vivo (108,123-125,128,129) suggest that the antimicrobial activity of nitrite depends not only upon the disruption of respiration [by destruction of the natural iron-sulfur clusters of redox proteins (70S)] but also specifically upon the formation... 

A series of comprehensive reviews7,9-11 documents the historical growth of nitrosyl chemistry and also shows the recent expansion that has taken place both in the more traditional areas and in the comparatively new organometallic branch12 of this subject. In this section it is our intention to discuss the structural aspects of nitrosyl coordination, to provide an account of the methods available for the preparation of nitrosyl complexes and to survey the reactions that occur at the nitrosyl group. 

The first thionitrosyl complex was discovered by chance during an attempted synthesis of molybdenum nitrido complexes in the presence of a source of sulfur, tetrathiuram sulfide.168 This work of Dilworth and Chatt was reported in 1974 and ultimately led to the syntheses of a range of Mo, Re and Os thionitrosyl complexes starting with the respective nitrido complexes.143,164 Nitrosyl complexes, like carbonyls, had been known for decades, and the more recent syntheses of thio-carbonyl complexes forebode the advent of thionitrosyl complexes. However, convenient synthetic routes based on analogies with syntheses of NO complexes were generally not available because the requisite precursors did not exist or were inconvenient to handle. Sulfiliminato complexes (M=N=SR2) of transition metals are as yet unknown. The chemistry of thionitrosyl complexes was the subject of a recent review.179... 

The adsorption of NO on morphologically well-defined samples has not been investigated in detail. On high-surface-area materials (509, 510), stable nitrosyl species are formed together with disproportionation products (N20 and N02) this complex chemistry renders the interpretation of the resulting (complex) spectra difficult (unpublished results). However, the... 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

B. A. Prenz and J. A. Ibers Structural chemistry of transition metal complexes (1) Five-coordination (2) Nitrosyl complexes. 

Dinuclear and tetranuclear complexes are also suitable starting materials, and much of the synthetic and substitutional chemistry of these species reflects the ability of the systems to sustain monomeric solvated dinitrosyl species such as [Fe(NO)2(solvent)2]+ or [Fe(NO)2(SR)(solvent)]. The latter tend to be formed in solvents having poor and the former in ones having good tt-acceptor capabilities. A review of nitrosyl complexes of iron-sulfur complexes is available. 

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