Resonance structures nitrosyl complexes

A large number of paramagnetic transition metal nitrosyl complexes have been studied using electron spin resonance (ESR) spectroscopy. Information on the electronic ground state can be derived from the g-value and the hyperfine coupling constants, and many [MLslNO)]" (see Table IV) and nitrosyl porphyrin complexes (99) have been studied in this way with a view to understanding their electronic structures. 

Nonheme ferrous centers in some metalloproteins react reversibly with NO forming nitrosyl complexes with S = 312 characterized by the g values of about 4.0 and 2.0 [51]. The EPR spectrum of the nitrosylated NorR (abacterial NO-responsive transcription factor, the enhancer binding protein) is typical of a d high-spin Fe NO", where the S = 5/2 iron is antiferromagnetically coupled to the NO (Fig. 5, [52]). This is confirmed by the X-ray, resonance Raman, MCD, Mossbauer spectroscopies, and DFT calculations. Similar structures were proposed for the classical complexes, [Fe(NO)(l-isopropyl-4,7-(4-ferf-butyl-2mercaptobenzyl)-l,4, 7-triazacyclononane)], [53], Fe(EDTA)NO [54—56], the brown-ring compound, Fe(H20)5N0 [57], and for the Fe(N/V ,N -trimethyl-l,4,7-triazacyclononane) (N3)2N0 [54]. Interestingly, for the latter a spin equilibrium between the valence tautomers 5=1/2 and 3/2 in the solid state was observed. 

Fe—NO complexes also exhibit unique electronic structures due to the noninnocent nature of the nitrosyl ligand. The electrons in the Fe—NO bond are highly delocahzed and thus assignment of oxidation state in these systems is challenging and resonance structures are often proposed. For example, a complex may be described as a resonance hybrid between Fe(II)-NO <-> Fe(III)—NO a total of six electrons in the Fe 3d and NO Jt orbitals. In light of the extent of electron delocalization and difficulty in assigning oxidation state, Enemark and Feltham devised the generic notation (or EF notation) MNO, where x is the total sum of the Md and... 

The structure of CpCr(NO)2 is remarkable in that it demonstrated unequivocally for the first time the existence of bent M—N—O bonds in a nitrosyl complex. Although bent M—N—O bonds have been claimed in Co(NO)(S2CNMe2)2 ( ) and in a number of ruthenium complexes (48) the evidence is not conclusive. Electron spin resonance measurements on... 

The structural and synthetic diemistry of aryldiazenato-complexes have been described in the literature and similarities between the nitrosyl and aryldiazenato-complexes of transition metals have been noted. These similarities are particularly evident when resonance structures of linear (a), bent (b), and bridged (c) complexes are examined. Scheme 8.5. Analogous reactivity patterns for nitrosjd and aryldiazenato-complexes are also observed and this extends to redox reactions in which nitrosonium and aryldiazonium salts participate. 

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

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