Other Metal Nitrosyls

The dual role of these ruthenium complexes, as both an NO donor and scavenger, appears to lie strongly in the binding constant of NO to Ru(II) and the ligands present, which can facilitate NO dissociation. Clearly the investigative approach into these systems has just touched the surface and the future appears very promising in these potential NO source/scavenger systems. 

Manganese nitrosyl porphyrins [215] are considered good models for the iron-nitric oxide analogs, which are relatively unstable but very vital to many biological operations. A six-coordinate manganese nitrosyl porphyrin of the form (por)Mn(NO)(L), where por can be TTP (TTP = tetra(4-methylphenyl)porphine) and L = piperidine, methanol, 1-methyhmidazole, has been prepared [216] in moderate yields by the reductive nitrosylation of the (por)MnCl complex with NO in piperidine. The crystal structures of these compounds give indication of a linear Mn-NO bond [215]. 

Examples of the osmium and iridium complexes are Os(PPh3)2Cl(NO) and Ir(PPh3)3(NO), respectively [216]. The osmium compound gave, on reaction with HC1, the first characterized complex with the feature of an N-coordinated HNO, Os(PPh3)2Cl2(HNO), which was confirmed by X-ray crystallography. On the other hand, the nitrosylated iridium compound gave the hydroxylamine complex [216]. 

The nitric oxide reduction of Cu(dmp)2(H20)2+ in aqueous media gives a Cu(II)-NO complex via an inner-sphere mechanism [216] (dmp = 2,9-dimethyl-l,10-phen- 

4 Greenwood, N. N., Earnshaw, A., Chemistry of the Elements, Pergamon Press, Oxford 1993, 2nd edn., p. 508 

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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). 

To a first approximation the nitrosyls behave like the alkyl complexes and present the case of a balance between five- and six-coordination in which the metal could be written in the formal oxidation state Co(III). But the situation is complicated by the possibility of v bonding and hence different contributions from the a and 77 bonds, together with changes in the configuration of the Co-NO group and in the spin state. For experimental evidence for v bonding in other metal nitrosyls see, e.g., Manoharan and Gray (121) and Cans et al. (75). 

The nitrosation mechanism of aliphatic amines by metal nitrosyl complexes was studied with various secondary amines and one primary amine (ethylamine) using pentacyanonitrosylferrate (sodium nitroprusside). Very little mechanistic information is available for reactions with other metal nitrosyl complexes (see also Sects. 2.3, 4.3, and 7.2). 

This is the second of the common oxidation states for iron and is familiar for ruthenium, particularly with Group 15-donor ligands (Ru probably forms more nitrosyl complexes than any other metal). Osmium(II) also produces a considerable number of complexes but is usually more strongly reducing than Ru". 

Ruthenium probably forms more nitrosyl complexes [115] than any other metal. Many are octahedral Ru(NO)Xs systems, where X5 can represent a combination of neutral and anionic ligands these contain a linear (or very nearly) Ru-NO grouping and are regarded as complexes of ruthenium(II). They are often referred to as (Ru(NO) 6 systems. 

Iodine pentafluoride Metals Nitric acid Metals Nitrosyl fluoride Metals Perchloric acid Bismuth See other METALS... 

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. 

Some other reactions of metal nitrosyls LxM(NO) with various nucleophiles (Nuc) are summarized in Table III. The pattern indicated by the studies described above is repeated simple adduct formation occurs when the coordinated nitrosyls are sufficiently electrophilic and the nucleophiles sufficiently basic. The first species formed is probably the N-coordinated nucleophile nitrosyl adduct LrM(N(O)Nuc), e.g. Eq. (27). Subsequent reactions depend on the substitution lability of these species, as well as on the redox stability of the complex and of the ligand. 

The existence of copper-nitrosyl complexes of biological significance has been briefly discussed here (Section VII). It is worth pointing out that nitrosyl complexes of other metal-containing proteins may form, and that these may be important in understanding the effects of NO on living cells. Nitrosyl complexes of many other metals are well documented (e.g., Werner and Karrer, 1918 Moeller, 1952) and include complexes of nickel, cobalt, and ruthenium. Some such complexes may be less obvious than the paramagnetic and often colorful... 

Complexes of Other Metals. Having studied in detail catalysts derived from molybdenum nitrosyl complexes, it was interesting to investigate the effect on catalytic activity of substituting other transition metals in both nitrosyl derivatives and in other related complexes. 

Chlorine trifluoride Metals, etc. Dichlorine oxide Oxidisable materials Halogens, above Iodine pentafluoride Metals Iodine Metals Nitrosyl fluoride Metals Perchloric acid Antimony(III) compounds Potassium dioxide Metals Potassium permanganate Antimony, etc. Seleninyl chloride Antimony Sodium nitrate Antimony See other METALS... 

Since a far greater number of nitrosyl complexes have been isolated for ruthenium than for any other metal it might be expected that osmium should rival ruthenium in this respect. There seems no reason why this should not be so, and the present disparity in numbers of known osmium and ruthenium nitrosyls simply suggests that much less work has been carried out on the osmium systems. 

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