Nitrosyl metal interactions

The symmetry of transformation (107) is identical to that of the alkyl car-bonyl-to-acyl conversion, (5), and therefore the difference in the propensities of these insertions to occur must relate to differences in the orbital energetics of the two processes. Since the metal-nitrosyl jr-interaction is a dominant feature of the electronic structure in NO+ complexes, and since it will be weakened by the formation of a nitroso species, one can expect (107) to be correspondingly less favorable than acyl formation in (5). If reaction (107) does occur, however, one can readily envisage tautomerism of the nitroso ligand to an oxime. The reactive nature of the oxime may then create difficulties in the identification and isolation of organonitrogen products, and indeed, this may have obscured general observation of transformation (107) in the past. 

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). 

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. 

Figure 14. Frontier molecular orbital diagrams of (a) metal-oxo and (b) metal-nitrosyl tt-bonding interactions. [Adapted from (16).]...

Figure 30 Qualitative molecular orbital diagram illustrating the bonding in a linear metal nitrosyl species focusing on interactions with the d orbitals. The HOMO of NO is a singly occupied doubly degenerate orbital and its involvement in the bonding requires the metal to contribute three electrons to the derived molecular orbitals. Thus, a d metal center becomes d upon coordination of NO, as illustrated for the case in which n = 3. The interaction may, therefore, be represented via the triply bonded resonance structure M=N-0.

Figure 31 Different descriptions of the linear metal nitrosyl interaction. The oxidation number formalism implies that coordination of NO to a d metal center results in configuration, whereas a d configuration is implied by the CBC description.

In addition to donor interactions, due consideration must be given to molecules that feature multiple vr-acceptor ligands because it is possible that the metal may use the same orbital to backbond to more than one ligand. For example, consider a trans-M(NO)2 moiety (Figure 33). Focusing on the 7r-backbonding interactions, both nitrosyl... 

The impact of NO complexation by metal ions is of a crucial meaning to biological systems. NO reacts with all transition metals to give metal nitrosyls. Proteins containing transition metals are particularly prone to react with NO, since its unpaired electrrMi can interact and bond with the d-orbitals of metal cofactors. These interactions are widely used by the nature and function in various cellular regulatory pathways. It is clear that the diversity of the NO-sensing proteins... 

Moreover, selectivity could be also enhaneed if a specific interaction between the immobilized catalyst and the substrate in solution were to occur. A significant example is that of the voltam-metric detection of NO in the rat brain from a carbon fiber microelectrode modified with a [(H20)Fe PWii039] -containing poly(N-methylpyrrole) film and a Nation outer layer [150]. The good selectivity of this sensor was attributed not only to the Nafion membrane, which constituted an efficient electrostatic barrier against anionic interferents, but also to the formation of a metal-nitrosyl complex between the heteropolyanion and NO. The in vivo NO measurements were validated by injecting the rat with an NO-synthase inhibitor, which led gradually to the disappearance of the NO oxidation peak (Fig. 7). 

Henry YA, Singel DJ. 1996 Metal-nitrosyl interactions in nitric oxide biology probed by electron paramagnetic resonance spectroscopy. In Methods in nitric oxide research, pp. 357-372. Ed M Feelisch, JS Stamler. West Sussex John Whey Sons. 

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). 

The superhyperfine interaction is observed for metal complexes in cases where the metal ligands have a nuclear moment. For instance, the nitrosyl (NO) complexes of iron(II) heme proteins have two inequivalent axial nitrogen ligands. The 14N(/ = 1) NO couples strongly to the unpaired electron, yielding a widely split triplet with each component of equal intensity and separated by 2.1 mT. The second... 

A characteristic of the magnetic resonance spectra of nitrosyl complexes is the presence of hyperfine interactions between the unpaired electrons and the nuclear spin of the nitrogen. The strength and orientation dependence of these interactions are determined by the geometry of the complex and the localization of unpaired electrons in ligand and metal orbitals. 

In studies of reaction pathways, nitrosyl radicals are frequently used as spin traps to provide evidence for free radical pathways. A caution in interpretation of these results is that the probe or products will interact with the transition metal complex in the reaction and affect the reactivity of the probe with the organic substrate or free radicals produced. A number of reactions of the stable free radicals RNO and R2NO with platinum(II) complexes have been carried out which show that such reactions must indeed be considered (equations 473-... 

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