Nitric oxide transition metal nitrosyl complexes

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

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. 

Nitric oxide rapidly reacts with transition metals, which have stable oxidation states differing by one electron (see Chapters 2 and 3). Nitric oxide is unusual in that it reacts with both the ferric (Fe " ) and ferrous forms (Fe " ) of iron. TTie unpaired electron of nitric oxide is partially transferred to the metal forming a principally ionic bond. Complexes of ferric iron with nitric oxide are called nitrosyl compounds and will nitrosate (add an NO group) many compounds, while reducing the iron to the ferrous state (Wade and Castro, 1990). 

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),... 

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. 

A description involving two extreme bonding formulations is currently used to explain the nature of the M—N—O linkage in most mononuclear complexes. In the sense of formal oxidation state it is now customary to think of linear nitrosyls as derived from NO+ and bent (120°) nitrosyls as derived from NO-.23,24 Bond formation between free nitric oxide and a transition metal ion must then be considered to involve either prior donation of an electron from NO (giving NO+) or prior acceptance of an electron by NO (giving NO-) 25 in each case this would of course be followed by lone pair donation from the nitrogen. Using this somewhat simplistic view it is easy to explain the... 

Nitrosyl complexes are included in this section since, as Feltham and Enemark have noted, the predominantly covalent interactions between nitric oxide and transition metals make assignment of... 

One of the earliest assignments of an oxidation state of +1 for an iron complex was that of Wilkinson and co-workers85 based on the magnetic susceptibility, IR and absorbtion spectra of [Fe(H20)5N0]2+. The potential for nitric oxide to act as either a two or three electron donor in its coordination to transition metal ions, however, has led to much controversy. For this reason iron nitrosyl complexes are discussed separately in Section 44.1.2. 

The catalytic implications of the alternative coordination modes of nitric oxide in transition metal complexes were first noted by Collman (12). He argued that the linear bent transformation, concomitant with a change in the formal oxidation state of nitrogen from (III) to (I), results in the withdrawal of electron density from the metal center and facilitates the coordination of another ligand into a vacant site. Thus, the mixed carbonyl nitrosyl complex [Co(CO)3(NO)] undergoes thermal CO substitution by an associative mechanism, whereas the iso-electronic, homoleptic carbonyl [Ni(CO)4] reacts by a dissociative pathway (13). 

The discovery that the nitrosyl ligand is capable of binding to transition metals in two isomeric valence forms1 is one of the most dramatic recent developments in organometallic chemistry. Since bent NO donates 2 fewer electrons to the metal than the linear isomer does, linear-bent tautomerism raises the possibility of coordinative unsaturation and catalysis.2 In fact, complexes of nitric oxide are receiving increasing attention as catalysts,3 since they are more reactive than the corresponding carbonyls.4... 

A nitric oxide (NO) QD-based sensor was developed via the NO-stimulated ligand substitution of a transition metal complex assodated with CdSe/ZnS QDs [142]. The red-colored tris-(N-(dithiocarboxy) carcosine) iron(III)[Fe(DTCS)3] was linked to ammonium-capped QDs by ionic interaction. As a consequence, the functionalized QDs readed with NO by an ET process, followed by ligand substitution to yield the colorless paramagnetic bis(dithiocarbamato) nitrosyl iron(I) complex as a capping layer. This process triggered the luminescence of the QDs that enabled the detection of NO (Figure 6.11). 

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