Metal nitrosyls ligands

The nitro(ly)sation of thiol compounds goes directly via N2O3 and indirectly through metallic nitrosyl ligands ... 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

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. 

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 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]. 

Method (i) is a route commonly utilized in monometal nitrosyl complexes. The nitrosyl ligand may function as (formally) a three-electrop donor (NO+) with a linear bonding mode, or as (formally) a one-electron donor (NO ) with a bent (—120°) M-N-0 arrangement. Conversion of the M-NO system to a M-NO system has two effects. First, it increases the metal oxidation state by two second, it generates a vacant coordination site. The dinitrosyl cluster Os3(CO)8(NO)2, which has... 

The carbyne ligand may be viewed as a three-electron donor, similar to the nitrosyl ligand, with a pair of electrons in an sp orbital and a single electron in a p orbital. Donation of the sp electrons and pairing the p electron with one from the metal atom gives a a bond and a 7r bond, respectively. The second rbond results from donation of an electron pair from the metal atom to the empty p orbital of the ligand. 

Although the ultimate fate of the nitrosyl ligands has not been established, a reasonable pathway for their removal can be postulated based on our IR data and recent literature results. Shriver et al. (42) have found that organoaluminum species will complex with the oxygen of metal-coordinated carbonyl groups. For example, the six-coordinate... 

The structural studies of nitrosyls have shown to date that the bent nitrosyl ligand invariably occurs at the apical position of a square based pyramid or a distorted octahedron in which the metal ion configuration assuming NO- coordination is d6. Despite numerous electronic structural descriptions (168, 169,198-200), it is not totally clear why fully bent nitrosyls with M—N—O bond angles of 120° have not been found in other geometries such as the square plane and the trigonal bipyramid. 

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