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Nitrosyl complexes electronic structure

In contrast, for the NO8- species the N—O bond is elongated, only slightly polarized, and the stretching frequency, vNO, decreases below 1850 cm-1. Such changes indicate that the activation consists in redistribution of the electron and spin densities within the M—NO unit, which accumulates on the nitrogen atom. Among the first series TMI, the oxidative adsorption is less common and includes only the tj1 CuNO] 11 and 171 3CrNO 6 adducts. The mechanistic implications of the electronic structures for both type of the nitrosyl complexes are discussed in the next section. [Pg.51]

Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25). Table III also shows the values of the equilibrium constants, KVAp for the conversion of iron nitrosyl complexes into the corresponding nitro derivatives. Keq decreases downwards, meaning that the conversions are obtained at a lower pH for the complexes at the top of the table. Thus, NP can be fully converted into the nitro complex only at pHs greater than 10. The NO+ N02 conversion, together with the release of N02 from the coordination sphere, are key features in some enzymatic reactions leading to oxidation of nitrogen hydrides to nitrite (14). The above conversion and release must occur under physiological conditions with the hydroxylaminoreductase enzyme (HAO), in which the substrate is seemingly oxidized through two electron paths involving HNO and NO+ as intermediates. Evidently, the mechanistic requirements are closely related to the structure of the heme sites in HAO (69). No direct evidence of bound nitrite intermediates has been reported, however, and this was also the case for the reductive nitrosylation processes associated with ferri-heme chemistry (Fig. 4) (25).
The principal objective of the earliest X-ray studies to be carried out on iron-sulfur-nitrosyl complexes, those on [Fe2(SEt)2(NO)4] (10) and Cs[Fe4S3(NO)7] (11), was the establishment of their gross chemical constitution. More recent X-ray studies have been concerned not only with gross structure, but additionally with detailed comparisons within series of similar species as a possible probe of electronic structure. [Pg.354]

The symmetry of transformation (107) is identical to that of the alkyl car-bonyl-to-acyl conversion, (5), and therefore the difference in the propensities of these insertions to occur must relate to differences in the orbital energetics of the two processes. Since the metal-nitrosyl jr-interaction is a dominant feature of the electronic structure in NO+ complexes, and since it will be weakened by the formation of a nitroso species, one can expect (107) to be correspondingly less favorable than acyl formation in (5). If reaction (107) does occur, however, one can readily envisage tautomerism of the nitroso ligand to an oxime. The reactive nature of the oxime may then create difficulties in the identification and isolation of organonitrogen products, and indeed, this may have obscured general observation of transformation (107) in the past. [Pg.155]

Fig. 6. Molecular structure of the cation in the 19 valence electron nitrosyl complex [(l)Fe(NO)]Br2 (34). Fig. 6. Molecular structure of the cation in the 19 valence electron nitrosyl complex [(l)Fe(NO)]Br2 (34).
This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. [Pg.203]

The normal classification of material by oxidation state is inappropriate for nitrosyl complexes because the oxidation state concept is very much a formalism for them. Instead we shall use the generally accepted [M(NO)x] + classification in which x is the number of coordinated NO groups and n the number of metal d electrons, the latter being calculated on the basis that NO+ is the coordinated moiety. As will be apparent, osmium complexes within each such category do in fact show considerable similarities of structure and reactivity, and also with their ruthenium analogues. Osmium is unusual in forming an [M(NO)]5 type of complex. [Pg.544]

The molecular structure of [FeH(NO)(PF3)3] (16) has been proposed to be based on a trigonal bipyramid with the H and NO ligands occupying axial positions and the PF3 groups likely to be distorted toward the small hydride. The molecular structure of [Rh(NO)(PF3)3] (17) has been determined in the gas phase by an electron diffraction study (46) and has the expected C3 symmetry with an Rh-P distance of 2.245(5) A. Nitrosyl complexes containing PF3 are listed in Table XIV. [Pg.111]


See other pages where Nitrosyl complexes electronic structure is mentioned: [Pg.27]    [Pg.37]    [Pg.77]    [Pg.780]    [Pg.204]    [Pg.34]    [Pg.379]    [Pg.226]    [Pg.238]    [Pg.571]    [Pg.680]    [Pg.56]    [Pg.155]    [Pg.65]    [Pg.67]    [Pg.75]    [Pg.201]    [Pg.212]    [Pg.373]    [Pg.105]    [Pg.116]    [Pg.152]    [Pg.203]    [Pg.300]    [Pg.117]    [Pg.631]    [Pg.117]    [Pg.631]    [Pg.207]    [Pg.210]    [Pg.215]    [Pg.220]    [Pg.240]    [Pg.1243]    [Pg.783]    [Pg.850]    [Pg.1976]    [Pg.2135]    [Pg.2167]   
See also in sourсe #XX -- [ Pg.450 , Pg.451 ]

See also in sourсe #XX -- [ Pg.450 , Pg.451 ]




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