Metal-nitrosyl chemistry

Synthetic methods in transition metal nitrosyl chemistry. K. G. Caulton, Coord. Chem. Rev., 1975,... 

Recent developments in transition metal nitrosyl chemistry. N. G. Connelly, Inorg. Chim. Acta. Rev., 1972, 6, 47-89 (395). 

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). 

In their extensive studies of metal nitrosyl chemistry, Enemark and Feltham showed that the mode of NO bonding can be altered by the simple addition of another ligand (168, 204, 205). An example of this phenomenon is illustrated by reaction (83b), and has been described by these investigators as stereochemical control of valence (204). 

The propensity of the linearly bonded nitrosyl group to undergo nucleophilic attack is inversely related to the effectiveness of back-donation in metal nitrosyl n-bonding. Below a vN0 value of ca. 1860 cm-1, the nitrosyl is unreactive with nucleophiles. Above that value, and especially above 1900 cm-1, the nitrosyl ligand reacts with a variety of nucleophilic reagents at the N atom (171). This type of attack is the most widely known and best studied in metal nitrosyl chemistry. 

In this chapter, recent results are discussed In which the adsorption of nitric oxide and its Interaction with co-adsorbed carbon monoxide, hydrogen, and Its own dissociation products on the hexagonally close-packed (001) surface of Ru have been characterized using EELS (13,14, 15). The data are interpreted In terms of a site-dependent model for adsorption of molecular NO at 150 K. Competition between co-adsorbed species can be observed directly, and this supports and clarifies the models of adsorption site geometries proposed for the individual adsorbates. Dissociation of one of the molecular states of NO occurs preferentially at temperatures above 150 K, with a coverage-dependent activation barrier. The data are discussed in terms of their relevance to heterogeneous catalytic reduction of NO, and in terms of their relationship to the metal-nitrosyl chemistry of metallic complexes. 

These synthetic methods are based on oxygen transfer reactions, which are an important general feature of transition metal nitrosyl chemistry. The following reactions illustrate this route 15, 42) ... 

Fig. 2 Nitric oxide signaling relevant to metal nitrosyl chemistry. Reprinted from [254], with the permission of the American Chemical Society, copyright 2012...

The non-innocent character of the NO ligand is a basic feature of transition metal nitrosyl chemistry. Indeed, this feature may account also for the fact that nitrosyl complexes are rarely found in applications of homogeneous catalysis. Namely, the... 

The general topic of transition metal nitrosyl chemistry has been ably treated by Johnson and McCleverty (48), and this more restricted survey owes much to that review. Here we shall briefly consider the nature of the metal-ligand link in organometallic nitrosyls, the preparative chemistry of the main classes of compounds, and the physical data which have been derived for them, laying particular stress on the more recent developments. 

Thus, in metal-nitrosyl chemistry, it needs to be considered that Unkage isomerism, including formation of side-on // -NO metal ligation, may occur as a general phenomenon. This may derive from structural/electronic preferences, dynamic behavior or via photochemical initiation. Reactivity patterns and mechanisms, leading from or to M-NO moieties, new or old, should be considered in this light, e.g. as in the Cu-NIR case. 

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. 

In contrast to nitrosyls, the absence of a transferable oxygen atom in N2R ligands allows the preparation of stable diazenido complexes of oxophilic, early transition metals see for example Cp2TiCl(N2Ph). Furthermore, there are as yet no diazotate (RN=NO-) forming reactions anolo-gous to the nitrite forming reactions in nitrosyl chemistry (see equations 112 and 113). 

The adsorption of nitric oxide is of further interest from the point of view of metal-nitrosyl interactions in inorganic chemistry. The bonding of nitrosyl ligands to transition metal centers in metal compounds has indicated that the NO ligand is amphoteric, i.e., it can be formally considered as N0+ or NO-(linear or bent) when bonding to a single metal center (16,17). 

Periodic Table to justify the leaving of such slighted elements for a more advanced course taken by students with special interests in inorganic chemistry. Similarly, I have passed over many interesting but somewhat exotic compounds of the more usual elements (for example, the nitrides of sulfur, metallic nitrosyl compounds, heteropoly acids, and interstitial carbides and nitrides). 

Methods in Enzymology are devoted to various aspects of -NO chemistry aud biochemistry. There are also books dealing with metal-nitrosyls and their electron paramagnetic resonance (EPR) spectroscopy as well as various biomedical aspects of In addition, a recent book on -NO, its chemistry,... 

NO is a fascinating diatomic radical in the context of coordination chemistry due to its notorious non-innocent behavior in transition metal complexes. For example, NO adducts of ferrous iron complexes could have electronic structures that vary all the way from a Fe(I)—NO to a Fe(III)—NO extreme with the Fe(II)—NO (radical) case being intermediate. This distinction is significant, as it can be expected that NO, NO (radical), and NO will show very different reactivities. However, characterizing the exact electronic structures of transition metal nitrosyls is difficult, which led to the establishment of the famous Enemark-Feltham [Fe—NO]"" notation (the superscript x... 

One of the most prominent features of ruthenium chemistry is that the metal forms numerous compounds containing the (RuNO) + fragment. Furthermore, the metal-nitrosyl fragment has in general been of interest to many because of the potential 7r-acceptor ability of NO and its possible existence as NO+, NO , or NO. Explicit details of how to synthesize the ruthenium-NO complexes are virtually nonexistent, and procedures, when available, are laborious. Here we give the preparative descriptions for such compounds by newly developed methods which are explicit and convenient. The information is also valuable in view of the cost of ruthenium. 

The chemistry of transition metal nitrosyls has been reviewed, with spectra of many types used to study the electronic structure. Bonding, as described in Chapter 13, can be thought of as a linear complex of NO, isoelectronic with CO and with NO stretching frequencies of 1700 to 2000 cm or a bent complex of NO , isoelectronic with O2 and with NO stretching frequencies of 1500 to 1700 cm . The number of electrons on the metal ion and the influence of the other ligands on the metal provide for changes from one to the other during reactions. 

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. 

Reduction of metal-nitrosyl complexes is a prominent subject in the redox chemistry of small nitrogenated molecules, most relevant to biochemistry and to natural nitrogen-redox cycles.11 MnNO+ complexes have low-energy LUMOs and undergo facile one-electron reduction, usually showing reversible CV waves associated with MNO+/MNO redox couples.4"-6 The MNO 6 complexes yield fairly stable MNO 7 species in solution, as evident from UV-vis, IR, and EPR. Some of the latter species release NO in the minute time scale alternatively, the [Fe(CN)5NO]3 and [Ru(NH3)5NO]2+ ions may lose cyanide or ammonia.4"-6 Some metallopor-phyrins also afford reversible conversion between MNO1 / MNO forms.21 The NiR enzymes release NO after nitrite coordination and reduction.11... 

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