Transition metal nitrosyl complexes reactions

Until recently most of the mechanistic studies on nitrosation have been concerned with N-nitrosation reactions of amines, including the diazotisation reactions of primary amines. Now, work has been extended to include both O- and S-nitrosation, so that comparisons can be made. Mechanistic studies have also been extended in recent years to include reactions of nitrogen oxides, nitrosamines, alkyl nitrites, thionitrites and transition metal nitrosyl complexes. Many of these reactions have been used preparatively for a long time, but little has been known about their detailed reaction mechanisms. 

This chapter describes the synthesis of transition metal nitrosyl complexes with particular reference to routes that involve common reagents (e.g., NO and NOBF4). Methods for their characterization by spectroscopic and structural techniques are critically reviewed. The application of NMR for distinguishing among linear, bent, and bridging nitrosyls are emphasized. The bonding in metal nitrosyl complexes is reviewed from a molecular orbital and valence-bond point of view. Finally, the reactions of transition metal nitrosyl complexes are discussed. 

At present it is impossible to say whether Fig. 24 represents the correct mechanism because no intermediates have been detected. A mechanism for the reaction of the hyponitrite complex (53) with free NO is shown in Fig. 25. This mechanism is related to the reaction of free NO with transition metal nitrosyl complexes discussed earlier though it cannot account for the stoichiometric reaction with CO. 

The chemistry of transition metal nitrosyls has been reviewed, with spectra of many types used to study the electronic structure. Bonding, as described in Chapter 13, can be thought of as a linear complex of NO, isoelectronic with CO and with NO stretching frequencies of 1700 to 2000 cm or a bent complex of NO , isoelectronic with O2 and with NO stretching frequencies of 1500 to 1700 cm . The number of electrons on the metal ion and the influence of the other ligands on the metal provide for changes from one to the other during reactions. 

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),... 

We suggest that the initial step is heterolytic splitting of H2 or Dj, leading to the protonated or deuterated transition metal nitrosyl hydrides or deuterides, respectively. H/D exchange via the second nitrosyl ligand might lead to the mixed hydride-deuteride complex, which in a reverse reaction forms HD, and regenerates the cationic catalyst. 

Many methods have been used to prepare NO complexes. The reactions of transition metal complexes with gaseous NO or solutions of the dissolved gas have been used to form NO complexes. One example of such a reaction to form, a homoleptic nitrosyl complex is shown in Equation 4.30. Alternatively, the reactions of metal complexes with nitrosonium salts form metal-nitrosyl complexes. (Note Alcohol solvents should be used with caution, because they convert NO " to alkyl nitrites. In addition, tetrahydrofuran should not be used as solvent because it can be polymerized by NO". ) Two examples of such processes are shown in Equations 4.31 and 4.32. A range of organic nitroso compounds release "NO" under appropriate conditions and have been used to introduce NO into transition metal complexes. The generation of a metal-nitrosyl complex from F3CNO is shown in Equation 4.33. ° Inorganic nitrites have also been used to prepare NO complexes, ° as shown in Equation 4.34. ... 

In studies of reaction pathways, nitrosyl radicals are frequently used as spin traps to provide evidence for free radical pathways. A caution in interpretation of these results is that the probe or products will interact with the transition metal complex in the reaction and affect the reactivity of the probe with the organic substrate or free radicals produced. A number of reactions of the stable free radicals RNO and R2NO with platinum(II) complexes have been carried out which show that such reactions must indeed be considered (equations 473-... 

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