Metal/metallic nitrosyls complexes

Principles of structure, bonding and reactivity for metal nitrosyl complexes. J. H. Enemark and R. D. Feltham, Coord. Chem. Rev., 1974,13, 339-406 (126). 

Carter, O. L., McPhail, A. T. Sim, G. A. (1967) Metal-carbonyl and metal-nitrosyl complexes. Part V. The crystal and molecular structure of the tricarbonylchromium derivative of methyl benzoate, J. Chem. Soc. A, 1619-1626. 

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. 

IV. Examples from the Chemical Biology of Metal Nitrosyl Complexes... 

Nitrosonium (NO+) is a strong oxidant and the reduction potential to NO has been measured in non-aqueous media (1.67 V vs. SCE in CH3CN), and estimated for water (Eq. (3)) (12,15). NO+ is subject to rapid hydrolysis to nitrite (2H+ + N02 ), and therefore if formed in biological media would be short-lived. However, other less water-sensitive chemical species can act as NO+ donors in reactions leading to the nitrosation of various substrates. For example, the reactions of certain metal nitrosyl complexes with nucleophiles such as R SH can lead to the transfer of NO+ as illustrated in Eq. (4). Such reactions will be discussed in greater detail below. 

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]. 

A number of other metallic nitrosyl complexes have been examined as NO-donor drugs but none have proven to be of value [38]. 

There is interest in the possible use of other metal nitrosyl complexes as vasodilators, but from the series KJM(CN)6NO] where M = V, Cr, Mn, and Co (n = 3) or M = Mo (n = 4) neither the Cr nor the Mn complexes exhibit any hypotensive action (504). Iron-sulfur-nitrosyl clusters such as [Fe4S4(NO)4] are active, and their effects can be potentiated by visible light (505). 

Such metal-nitrosyl complexes have great physiological significance and are involved for example in enzyme regulation and protection against oxidation [62],... 

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. ... 

Nuclear quadrupole resonance spectroscopy, see also specific compounds cyclophosphazenes, 21 93 of subvalent compounds, 29 298, 308-309 transition metal nitrosyl complexes, 34 313-314... 

Reductive nitrosylation, transition metal nitrosyl complexes, 34 296-297 ReFejSj cluster, 38 41-43 self-assembly system, 38 41-42 Refining, of actinide metals, see Actinide, metals, purification Refractory compounds heat treatment of solids, 17 105-110 crystal growth, 17 105, 106 decomposition, 17 107,-110 spheroidization, 17 106, 107 preparation of, using radio-frequency plasma, 17 99-102... 

The literature is rich with examples of metal-nitrosyl complexes, and it would be surprising if the generation of NO by the immune system did not result in the formation of many such adducts. Previous articles have presented summaries of metal proteins that form NO complexes (Butler et al., 1985 Henry et al., 1993), and more recently evidence has mounted that generation of NO by the immune system and by endothelial cells produces a variety of iron-nitrosyl complexes (Mulsch et al., 1993 Vanin et al., 1993 Lancaster et al., 1994). It is unclear which of the potential products will prove to be of physiological relevance, but because the enzymes that may be involved range from the central focus of oxidative cellular metabolism (LoBrutto et al., 1983) to the enzymes of DNA repair (Asahara et al., 1989), the list of potential targets is long and varied. 

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. 

Heme—NO complexes are the best known and most thoroughly studied of the biological metal-nitrosyl complexes. So many heme enzymes have been... 

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