Iron structural types

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

The compound [Cl2SiFe(CO)4]2 is obtained from Fe(CO)j and HSiClj, in a sealed tube reaction (148). It is presumably a member of the family of Group IVB-iron metallocycles of structural type (XI) (40, 246). Two Ni... 

Garnet, yttrium iron garnet, cI160, structural type... 

A microbial strain may produce more than one siderophore. There are variations in fatty acid chains of a lipophilic part or in the amino acids making up the backbone, as well as released intermediates of the biosynthetic chain. These variations belong all to the same structural pattern. However, there is also the possibility that so-called secondary siderophores are encountered. They constitute a different structural type, usually less complex in their constitution but also less efficient in binding Fe " than the primary ones. Secondary siderophores will be produced when the demand for iron is not so severe or in case there is a genetic defect impeding the production of the primary ones. Examples will be found throughout the review. 

In addition to their varied biological roles, non-heme iron proteins contain a magnificent assortment of iron sites having a multitude of chemical and structural properties. Indeed, the catalog of iron centers is a bit like the taxonomy of insects—a seemingly limitless variation of a few structural themes, yet each new form sufficiently different to define a new species. It is beyond the scope of any review of non-heme iron proteins to be inclusive, and there are excellent recent reviews which detail selected topics. Rather, it is our intention to provide in one chapter an overview of the major classes with an emphasis on proteins for which a crystal structure is available. This review begins with a survey of the types of protein iron structures and a discussion of some methods and problems associated with establishing the iron center type. This should provide an introduction to readers less familiar with the area. Sections II to IV include the current status and recent developments for a limited number of proteins from the major iron classes. These have been chosen in the subjective vein of a limited review the omission of a topic does not indicate its relative importance or interest, only the limitation of space. The purpose of this section is to emphasize the diversity of iron center structures and functions. 

Fig. 1.12 The iron boride structure type. From Wells (1986).

A comparison of the volumes per formula unit for the trifluorides is more complicated than for the difluorides because of the changes in structure type. If these changes are ignored, then some correlation with electronic configuration is possible. For the first transition series the volume decreases from scandium to chromium, increases from chromium to iron, and decreases to cobalt. This is as expected for the characteristic ligand-field stabilization effects (49), comparable with those for the difluorides. In the second series, there is a decrease from... 

Germanium and tin analogs of structures (LXXVI) and (LXXVII) are known (172,213,263,414), and some of them are fluxional (4,213) a germanium analog of compound (LXXVIII) has also been reported (77 see also Ref. 324). No silicon-iron example of the structural types represented by (LXXIX) (77) or (LXXX) (320) has yet been prepared,... 

The spectra illustrated in fig. 5.15 show that absorption maxima of all spin-allowed CF bands move to longer wavelengths with increasing iron content of the orthopyroxene, forming the basis of composition determinative curves (Hazen et al., 1977b Adams, 1974) and enabling this pyroxene structure-type to be identified in telescopic reflectance spectra of surfaces of the Moon (Pieters etal., 1985 Bums, 1989a). 

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