Sulfur-Metal Interactions

Dissolved sulfides (H2S and HS ), formed from sulfate reduction, can react with Fe(ll) and precipitate as iron sulfides (Berner, 1970). Under conditions of early diagenesis of amorphous iron sulfides (FeS), mackinawite (FeSg 9) is formed. One of the first minerals formed is hydrotrolite (FeS H20), which is labile under acidic conditions. During diagenesis the hydrotrolite converts to pyrite, which is a more stable mineral. The reaction can be simply represented as 

In the presence of elemental sulfur (S°), FeS is transformed to gregite (Fe3S4) and pyrite (FeSj)  

In most wetland or coastal systems, iron monosulfide is thermodynamically less stable than pyrite, but its formation is kinetically favored over pyrite. 

The formation of pyrite can occur following two mechanisms (i) single pyrite crystals are formed rapidly through direct precipitation of Fe(II) and polysulfides (S ) and (ii) framboidal pyrite is produced by a slower reaction of FeS with S° to produce a gregite intermediate (Equations 11.1 and 11.2). The direct precipitation of pyrite requires the prior oxidation of H2S to either S° or S for reactions like the following (Luther, 1991 Rickard and Luther, 1997)  

The two mechanisms for pyrite formation can proceed at the same time in the coastal marsh profiles. The rapid pyrite formation (Equation 11.3, 11.4, or 11.5) occurs in the upper 20 cm of marsh profile (more oxidized), displaying a larger variation in pyrite content, whereas the slow pyritization (Equations 11.1 and 11.2) tends to exist below 20 cm depth of marsh profile (more reduced) where maximum concentration of pyrite is normally found. 

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Fig. 6.35. Schematic energy-level diagram for the 3d orbitals in C09S8 showing a conduction band formed through metal-metal interactions between tetrahedral-site cobalt atoms [ct M(T)] and metal-sulfur-metal interactions to octahedral-site cobalt atoms [ct M(0)] (after Prewitt and Rajamani, 1974 reproduced with the publisher s permission).

Additional types of k C-k E chelating ylide complexes merit mentioning here, in addition to the species already presented, and some of them are shown in Scheme 24. The first is formed by ylides containing a pyridine functionality as substituent of the ylidic carbon. This type of ligand has allowed the S3mthesis of many loose clusters (82) of Cu, Ag, and Au which show weak metal( / °)-metal( / °) interactions. These facts have prompted the definition of a new phenomenon numismophilicity) to account for these weak interactions, uniquely shared by the three coinage metals [164]. Nitrogen and sulfur keto-stabilized ylides have been reacted with Pt and... 

Bonding concepts for cubanes, as discussed in Chapter 2.3., predict that the metal polyhedra in the cubanes will contract upon oxidation because of increased metal-metal interaction and vice versa. This has been verified for [CpFeS] 381), [CpFe(CO)]4 167, 177), and [CpCoS]4 361). Furthermore, the ease of oxidation and reduction of several cubane-type clusters 166,167, 361, 381), and the delocalization of electrons in the charged species 48, 176,177, 401) is noticeable. This, together with the prefered formation of iron-sulfur clusters, 381), is borne out by the fact that Nature uses iron-sulfur proteins for redox reactions 207). 

The rationale for studies on flavin semiquinone metal interactions stems from the presence of flavin coenzymes which participate in electron transfer in a number of metalloflavoproteins. Iron-containing redox centers such as the heme and nonheme iron sulfur prosthetic groups (Fe2/S2, Fe+ZS, or the rubredoxin-type of iron center) constitute the more common type of metal donor-acceptor found in metalloflavoproteins, although molybdenum is encountered in the molybdenum hydroxylases (e.g. xanthine oxidase, aldehyde dehydrogenase). 

It has been suggested that intermolecular copper—sulfur interactions in these complexes are of comparable magnitude to proton—sulfur and proton-metal interactions and should not be regarded as a general feature of the structural characteristics of the Cu(R2Dtc)2 complexes. It should be noted that in the structure of the Cu(PyrrolDtc)2 complex, the crystal packing is domin-... 

Mechanical activity at the surface such as load, speed, and variations in surface energetics, play a role in surface chemistry. For example, if a clean metal surface is exposed to materials such as oxygen, chlorine, and sulfur, an interaction goes on. There is no activation energy necessary to achieve the reaction of the species with the metal surface to form surface compounds. 

Grey selenium (mp 494 K, metallic) is the stable form. It may be obtained crystalline from hot solutions of Se in aniline or from the melt. The structure, which has no sulfur analogue, contains infinite, spiral chains of selenium atoms. Although there are fairly strong single bonds between adjacent atoms in each chain, there is evidently weak metallic interaction between the neighboring atoms of different chains. Selenium is not comparable with most true metals in its electrical conductivity in... 

Copper Proteins Oxidases Copper Proteins with Dinuclear Active Sites Copper Proteins with Type 1 Sites Copper Proteins with Type 2 Sites Cytochrome Oxidase Iron Heme Proteins Dioxygen Transport Storage Iron Heme Proteins Electron Transport Iron Heme Proteins, Mono- Dioxygenases Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases Iron Proteins with Dinuclear Active Sites Iron Proteins with Mononuclear Active Sites Iron-Sulfur Proteins Peptide Metal Interactions Zinc DNA-binding Proteins Zinc Enzymes. 

It is well known that cyclopentenes are coke precursors [4], because of their fast polymerization over the metal particles without desorption [11]. Therefore, it can be assumed that sulfur restrain the polymerization of the dehydrogenated cyclocompounds by weakening the double bond-metal interaction and favoring the desorption of the MCP . Similar effects... 

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