Nitric oxide metal nitrosyl complexes

Carbonyl Nitric Oxides. Another group of metal-carbonyl complexes, worthy of investigation as CVD precursors, consists of the carbonyl nitric oxides. In these complexes, one (or more) CO group is replaced by NO. An example is cobalt nitrosyl tricarbonyl, CoNO(CO)3, which is a preferred precursor for the CVD of cobalt. It is a liquid with a boiling point of 78.6°C which decomposes at 66°C. It is prepared by passing NO through an aqueous solution of cobalt nitrate and potassium cyanide and potassium hydroxide. ... 

Ford, P.C., Bourassa, J., Miranda, K. et al. (1998) Photochemistry of metal nitrosyl complexes. Delivery of nitric oxide to biological targets, Coord. Chem. Rev., 171, 185. 

Fe(CN)5(NO)] makes several appearances in a recent symposium volume dedicated to nitric oxide in biosystems, for example in cormection with iron(II) dtrate-induced oxidative stress." " Nitroprusside is the only metal nitrosyl complex in clinical use, where it is important as a rapidly acting agent for the lowering of exceptionally high blood pressure. ... 

Nitric oxide can bind to metals in both terminal and bridging modes to give metal nitrosyl complexes. Depending upon the stereochemistry of the complexes, NO may exhibit within one given complex either NO+ or NO-character, as illustrated in Table 15.2.2. 

In the presence of metal catalysts nitrosations using nitric oxide proceed rapidly and it is clear that a very powerful nitrosating species is produced. Rate measurements on the reaction of diethylamine with nitric oxide in the presence of Cu(II) salts indicated that a copper-nitrosyl complex was that species (Brackman and Smit, 1965). Many metal-nitrosyl complexes are now known... 

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. 

Still more problematic is the apparent migratory insertion of nitric oxide into transition metal-carbon bonds, an important reaction in metal nitrosyl complexes and one that may be relevant to biochemical reactions (7). On the evidence of isotopic labeling and kinetic experiments, the insertion of NO into the C0-CH3 bond of the (cyclopentadie-nyl)cobalt complex (i75-C5H5)Co(NO)(CH3), which occurs in Reaction (3),... 

The first accurately documented example of a distinctly bent M-N-0 linkage in a metal nitrosyl complex was presented in 1968. Ibers showed that the cation [IrCl(CO)NO(PPh3)2] (Figure 2) possessed a square pyramidal structure with a metal-nitrosyl bond angle of 124°. Subsequently other species exhibiting similar structural properties have been reported. It has now become clear that there are essentially two distinct orientations in which nitric oxide may be complexed. In one case the ligand is coordinated linearly and in the other it is bent (angle approximately 120°) for structural data available on linear and bent nitrosyls see Tables 1 and 2 respectively. 

Wang YX, Legzdins P et al (2000) Vasodilator effects of organotransition-metal nitrosyl complexes, novel nitric oxide donors. J Cardiovasc Pharmacol 35 73-77... 

Gans P (1965) The bonding of nitric oxide in transition-metal nitrosyl complexes. Chem Commun (London) 144—145... 

The first volume starts with an introductory chapter by myself on Historical Introduction to Nitrosyl Complexes recounts the discovery of nitric oxide and its complexes and serves as a general broad introduction to the two volumes. This is followed by a pair of chapters by Dr. Hanna Lewandowska on the Coordination Chemistry of Nitrosyls and Its Biological Implications and the Spectroscopic Characterization of Nitrosyl Complexes. A comprehensive overview is presented of the biologically relevant coordination chemistry of nitrosyls and its biochemical consequences in the first chapter. Representative classes of metal nitrosyls are introduced along with the structural and bonding aspects that may have consequences... 

Nitric oxide serves important roles in mammalian bioregulation and immunology. This has prompted research into metal nitrosyl complexes, which display low thermal reactivity but which are photochemically active to release NO and potentially deliver it to biological targets. One such complex that has been studied with ns-TRIR in the z/(NO) region and also conventional flash photolysis is Roussin s black salt anion, [Fe4S3(NO)7] . The TRIR experiments identified an intermediate species with a z/(NO) that was shifted to lower frequency, relative to the parent complex. This was identified as the product of NO photodissociation from one of the Fe(NO)2 units. 

Transition metal compounds containing nitric oxide as a coordinated ligand are normally called nitrosyl complexes. However, the term nitrosyl , is only sometimes restricted specifically to complexes which can be regarded as containing a three-electron metal-nitric oxide bond, and the term seems to be used generally for all nitric oxide compounds. Although there are many transition metal-nitrosyl complexes, relatively few also contain metal-carbon linkages and therefore fall within the subject of this chapter. 

Metal nitrosyls are the transition metal complexes of nitric oxide (NO) containing a metal-nitrogen bond. Roussin s red salt, Na2[Fe2(NO)4S2], and Roussin s black salt, Na[Fe4(NO)7S3], were the earliest known metallic nitrosyls. In line with the inclusion of metal carbonyl complexes under the category of organometaUic compounds, the metallic nitrosyl complexes are also recently included in organometaUic compounds. The nitric oxide cation is isoelectronic with carbon monoxide. Hence, there is quite a bit of resemblance in the chemistry of metaUic carbonyls and nitrosyls. However, the contrasts in these chemistries are also noteworthy. 

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