Mossbauer spectroscopy nitrosyls

The Fe(II)-NO complexes of porphyrins 66-68) and heme proteins 24, 49, 53, 69-76) have been studied in detail by EPR spectroscopy, which allows facile differentiation between five-coordinate heme—NO and six-coordinate heme—NO(L) centers. However, only a few reports of the Mossbauer spectra of such complexes have been published 68, 77-82), and the only Fe(III)-NO species that have been studied by Mossbauer spectroscopy include the isoelectronic nitroprusside ion, [FeCCNlsCNO)] (7S), the five-coordinate complexes [TPPFe(NO)]+ 68) and [OEPFe(NO)]+ 82), and two reports of the nitro, nitrosyl complexes of iron(III) tetraphenylporphjrrins, where the ligand L is NO2 82, 83). 

Mossbauer Spectroscopy. Mossbauer spectroscopy has been used as a powerM technique to probe the electronic stracture of the five- and six-coordinate ferrous nitrosyl porphyrins. The isomer shifts of both types of complexes (5 0.35 mms ) are similar and show temperature... 

A number of transition metal nuclei can be studied by Mossbauer techniques (e.g., Fe, Ni, Ru, W, Os, Ir, and Pt). Of these, only Ir, Ru, and Fe have been used to study nitrosyl bonding. The most detailed studies have been on the well-known iron complexes [Fe(CN)5(NO)] - (87. 89) and [Fe(NO)(dtc)z] (88-90) (dtc is N,N-dialkyldithiocarbamato). In the latter, high-spin/low-spin equilibria can be followed by Fe Mossbauer spectroscopy, and the Mossbauer parameters agree well with data from electron spin resonance (ESR) spectroscopy in determining the ground states of these complexes. 

The EPR silent metalloporphyrins are considered to be low-spin Mm (S = 1 /2) coupled to NO (S=l/2), as suggested by Mossbauer spectroscopy.19,20 The first characterized iron derivative [Fe(OEP)(NO)(OH2)]+15 and other related complexes have short Fe—N(NO) lengths (ca. 1.65 A), consistent with a strong bond (one a and two ir). The iron is located on the center, close to the porphyrin plane. Interestingly, NO is labile in the porphyrin-Fem systems, in contrast with the substitution-inert complexes with classical coligands, as nitroprusside (NP). Although these ferriheme derivatives are difficult to isolate because of reductive nitrosylation (see Section 1.31.1.5), some [Fe(OEP)(NO)(L)]+ complexes have been characterized. Figure 2 shows the structure of the L= l-Melm derivative. 

Iridium(V) complexes, 1158 fluorides, 1158 Iridium(VI) complexes, 1338 Iron complexes acetonitrile, 1210 analysis, 1180 biological systems, 1180 coordination geometries, 1183 coordination numbers, 1182-1187 dinitrosyldicarbonyl, 1188 MOssbauer spectroscopy, 1181 nitric oxide, 1187-1195 nitrosyls binary, 1188 bis(dithiolene), 1193 carbonyl, 1188 dithiocarbamates, 1192 halides, 1193 iodide, 1193... 

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. 

Application of Mossbauer Spectroscopy in the Investigations of Some Transition Metal Nitrosyl Complexes... 

The MB spectroscopy is apphcable in investigating the stmctures of metal nitrosyls, as MNO electronic stmcmre is extremely sensitive to its coordination environment. It is particularly effective when the spectroscopic measurements are combined with the results of DFT calculations. The application of the MB parameters in the isomer shift calibration allows one to precisely estimate the Fe and NO spin populations (see, e.g., [121]). Recently DFT has become an increasingly popular tool to calculate structures and Mossbauer properties [118, 138]. MB... 

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