Nitrosyl complexes coordination modes

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... 

Alkyl nitrites have also been used in nitrosyl synthesis and their mode of action may be explained by analogy with the above. One can consider that initial coordination of RONO is quickly followed by H+ attack giving a free alcohol and the nitrosyl complex (equation 12). 

Reactions have been known to occur at the nitrosyl group in the nitroprusside ion, [Fe(CN)3-NO]2-,8 for some considerable time recently similar behaviour has been observed in other systems. Different types of reactivity are exhibited depending on the nature of the nitrosyl complex and the mode of coordination of the NO ligand. For general reviews of this topic see references 9, 11 and 14. 

As ontlined below, NRVS stndies have provided information on iron ont-of-plane modes that was previously unavailable. For five-coordinate nitrosyl complexes, classical theory predicts that the three-atom FeNO grouping should have three modes the N-O stretch, the Fe-N stretch, and the FeNO bend, with the latter two expected to be found in the region below 600 cm. In a detailed analysis of [Fe(TPP)(NO)] provided by a series of DFT calculations, the predicted frequency of the stretch was overestimated when the calculation used the BP86 functional and underestimated when the B3LYP functional was employed. The reasons for the difficulties in this prediction appear to arise from the unusual nature of the unpaired electron in the compound that is significantly delocalized over the NO ligand as well as the metal. In this case, the Fe-N stretch was actually observed at 540 cm for [Fe(TPP)(NO)] powder at 80 This... 

Although the bis(dithiocarbamate) complexes of Fe(II) are relatively unstable to air oxidation, early studies (12, 15) produced stable adducts of NO and CO. Both 5-coordinate [Fe(NO)(I dtc)2] and 6-coordinate [Fe(NO)2(R2dtc)2] complexes are known. There has been considerable interest in the mode of attachment of the NO molecule, as there are six possibilities (see Scheme 4). (A) and (B) represent valence-bond structures of the linear Fe-NO bond. Structure (C) involves a symmetric Fe-NO TT-bond. Structure D illustrates the bent mode of attachment, in which nitrosyl is coordinated to the metal through the nitrogen atom, but the Fe-NO bond-angle differs greatly from 180°. Structures (E) and (F) are valence-bond formalisms of an unsymmetrical, metal-NO 7r-bond. The structure of [Fe(NO)(R2dtc)2] (R = Me or Et) has been shown (230,231) to be square pyramidal, with four sulfur atoms in... 

Although not an organic ligand, the NO (nitrosyl) ligand deserves discussion here because of its similarities to CO. Like CO, it is both a a donor and tt acceptor and can serve as a terminal or bridging ligand useful information can be obtained about its compounds by analysis of its infrared spectra. Unlike CO, however, terminal NO has two common coordination modes, linear (like CO) and bent. Examples of NO complexes are shown in Figure 13-18. 

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). 

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. 

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