Nitrosyl-metal complexes Subject

The coordinative unsaturation of some organo-lanthanide species can be overcome by the insertion of transition metal complexes containing donor atom ligands such as for example nitrosyl or carbonyl groups. The first studies on the subject deal with the... 

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. 

Ligand substitution reactions of NO leading to metal-nitrosyl bond formation were first quantitatively studied for metalloporphyrins, (M(Por)), and heme proteins a few decades ago (20), and have been the subject of a recent review (20d). Despite the large volume of work, systematic mechanistic studies have been limited. As with the Rum(salen) complexes discussed above, photoexcitation of met allop or phyr in nitrosyls results in labilization of NO. In such studies, laser flash photolysis is used to labilize NO from a M(Por)(NO) precursor, and subsequent relaxation of the non-steady state system back to equilibrium (Eq. (9)) is monitored spectroscopically. 

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]. 

The first thionitrosyl complex was discovered by chance during an attempted synthesis of molybdenum nitrido complexes in the presence of a source of sulfur, tetrathiuram sulfide.168 This work of Dilworth and Chatt was reported in 1974 and ultimately led to the syntheses of a range of Mo, Re and Os thionitrosyl complexes starting with the respective nitrido complexes.143,164 Nitrosyl complexes, like carbonyls, had been known for decades, and the more recent syntheses of thio-carbonyl complexes forebode the advent of thionitrosyl complexes. However, convenient synthetic routes based on analogies with syntheses of NO complexes were generally not available because the requisite precursors did not exist or were inconvenient to handle. Sulfiliminato complexes (M=N=SR2) of transition metals are as yet unknown. The chemistry of thionitrosyl complexes was the subject of a recent review.179... 

Transition metal nitrosyl complexes have been the subject of several reviews.273"277... 

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... 

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. 

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