Reviews nitrosyl complexes

Substituted derivatives of nickelocene, where one ring has been replaced, include the complex cyclopentadienyl nitrosyl nickel [12071 -73-7], (7T-C3H3)NiNO, a red Hquid, mp -41°C. A review of nitrosyl complexes with nickel is available (89). The dimer complex... 

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]. 

As briefly alluded to, there are different classes of redox-active ligands in addition to the above mentioned ones. For example, we have seen in Chapter 5, Section 8, that azo-groups (in particular, 2-(phenylazo)pyr-imidine) are able to undergo two separate one-electron reduction processes. Conjugated polynitriles (mnt, tcne, tcnq) also constitute an important class of redox-active molecules and the electrochemical behaviour of their metal complexes has been reviewed.107 The same holds as far as alkyldithiocarbamates (Rdtc) and their metal complexes are concerned,108 or nitrosyl complexes in their possible NO+[NO fNO redox sequence.109 Thus, we would like to conclude the present Chapter by discussing a few less known redox non-innocent ligands. 

In a recent review (215), a qualitative MO scheme is presented for the iron nitrosyl complexes. The two major features in this scheme are that (1) the Fe- NO group is linear because the 3b and 3fe2 orbitals are not occupied and (2) the highest occupied (and slightly antibonding) orbital is 5a,. 

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). 

Reactions have been known to occur at the nitrosyl group in the nitroprusside ion, [Fe(CN)3-NO]2-,8 for some considerable time recently similar behaviour has been observed in other systems. Different types of reactivity are exhibited depending on the nature of the nitrosyl complex and the mode of coordination of the NO ligand. For general reviews of this topic see references 9, 11 and 14. 

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 higher stability of ferrous heme nitrosyl complexes compared to the corresponding ferric species is well documented [see Refs (44-46, 69) for reviews]. Since cytochrome P450, catalase, and cytochrome c oxidase are commonly associated with reducing agents, a mechanism exists for longer term inhibition of the activity of these proteins. 

The reductive nitrosylation of heme proteins (Eq. 12) is as important a detection method for HNO as oxidation of Mb02 is for NO [Eq. 1 (170)]. Under anaerobic conditions, the nitrosyl complexes of reduced heme proteins are generally quite stable [reviewed in (44 -6, 69)]. However, in the presence of 02, these species decompose back to the starting material by the following mechanism [for a discussion see Ref. (44)]. 

There are six principal routes available to prepare diazenido complexes and these have been reviewed before (143, 154, 275, 317), although for convenience the relevant routes will be outlined here. Other routes such as the reaction of nitrosyl complexes with an aniline (41), or the reactions of 1,3-diaryltriazenes in acid (ready cleavage to yield diazonium ions in situ) with transition metal complexes (317), are of much less general utility. 

Transition metal nitrosyl complexes have been the subject of several reviews.273"277... 

There are several recent and comprehensive reviews on metal nitrosyl complexes which give good coverage to the osmium complexes 198 a review on ruthenium nitrosyl complexes is particularly... 

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. 

This chapter describes the synthesis of transition metal nitrosyl complexes with particular reference to routes that involve common reagents (e.g., NO and NOBF4). Methods for their characterization by spectroscopic and structural techniques are critically reviewed. The application of NMR for distinguishing among linear, bent, and bridging nitrosyls are emphasized. The bonding in metal nitrosyl complexes is reviewed from a molecular orbital and valence-bond point of view. Finally, the reactions of transition metal nitrosyl complexes are discussed. 

Although previous reviews have discussed the synthesis and reactions of nitrosyl complexes (1-11), they have not emphasized the characterizational aspects. Therefore, this review will attempt to correct this imbalance. 

In this section the reactivity of metal nitrosyl complexes is discussed and related to the NO coordination mode. A considerable difference between the chemistry of nitric oxide and carbon monoxide complexes has already been noted. The reactivity of nitric oxide coordinated to transition metal centers, and of nitrosyl clusters, were thoroughly reviewed in 1979 by McCleverty (7) and in 1985 by Gladfelter 11), respectively therefore only a summary is presented here. Nucleophilic reactions of linear nitrosyl groups will not be considered. 

Our discussion will comprise separately the mononitrosyl, polynitrosyl, bridging, and cluster nitrosyl complexes, with emphasis on the first group. We present different types of MNO compounds, without attempting to collect all available structures. Examples with either classical coligands (amines, cyanides, water, polypyridines, etc.) or porphyrin compounds have been chosen, in an attempt to illustrate recent developments in the field. Apart from the information contained in the previous Comprehensive Coordination Chemistry (CCC, 1987)17 a comprehensive review on structures... 

Other electrophiles, such as Li+ and BF3, have been shown to bind weakly to ComNO complexes, which react with oxygen-forming ComN02 complexes, able to oxidize alcohols catalytically. The activity of dioxygen and other oxidants as electrophiles toward nitrosyl complexes has been reviewed.3... 

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