Metal sulfide, electron transfer

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Khramov, M.I. and Parmon, V.N. (1993) Synthesis of ultrafine particles of transition-metal sulfides in the cavities of lipid vesicles and the light-stimulated transmembrane electron-transfer catalyzed by these particles. Journal of Photochemistry and Photobiology A-Chemistry, 71, 279-284. 

Our knowledge concerning soluble metal complexes with sulfide ions as ligands has increased considerably during the last two decades and this kind of Compound is still of topical interest. Some of the reasons for this are the development of a very flexible and fascinating structural chemistry of multinuclear metal-sulfur complexes, the fact that the active sites of some electron transfer proteins contain metal ions and labile sulfur,41,42 and also the relation of metal-sulfur cluster compounds to some heterogeneous catalysts. In addition, apart from the numerous binary and ternary sulfides which occur in nature, we have at our disposal a rich solid state chemistry of metal sulfides, which has been reviewed elsewhere and will be excluded here.43"17... 

Metal sulfides belong to the most important classes of compounds because they are of general significance for geochemistry (they are the most important ores for many metals), analytical and structural chemistry, and biochemistry (metal sulfide systems act as electron transfer systems) as well as catalysis (a high percentage of industrially used heterogeneous catalysts are sulfides) and materials science. 

The reductive decomposition of thiocyanato complexes should be applicable to the electrodeposition of other metal sulfides. We have tried this with Pd2, Co2+, Ni2+, Zn2+ and In3+.I8 While thin films of PdS, CoS and NiS could be successfully electrodeposited, other metal sulfides such as ZnS and In2S3 could not be obtained. This is an interesting series of results when we think of the softness (hardness) of these metals as acid. TC coordinates with its sof basic S atom to soft acidic Cd2+ and Pd2+, while hard acidic In3+ only permits coordination with hard basic N atom to form an isothiocyanato-complex. Other metals are at the borderline accepting coordination of both S and N. Because reduction of TC is catalyzed by a central metal,75,76) such ligand reduction may result in the formation of metal sulfides only for thiocyanato-complexes. The difference in bahavior among Co2+, Ni2+ and Zn2+ could be reasoned as the consequence of efficient catalysis of the electron transfer reaction by the transition metals. Such trends fit nicely with the previous findings by electrochemical analyses. 7) It is therefore understood that the chemical structure of the active species is decisive to the film formation. Thus, designing such molecular precursors which are chemically stable but can be electrochemically decomposed to metal sulfides should broaden the possibilities of electrochemical thin film synthesis. 

Iron is the most abundant metal on earth and the commonest electron transfer agents involve iron complexes. Life is thought to have evolved in reductive conditions, in which the dominant form of iron would be as iron sulfide, not iron oxide. The simplest forms of electron transfer agents (found in plants and bacteria) involve iron with thiolate ligands. Some simple electron transfer proteins, such as rubredoxin, contain a single iron centre in an S4 donor environment within a protein (Fig. 10-7). 

Oxidative dissolution of metal-sulfides (e.g., pyrite, FeS2) is a complex process involving surface adsorption of the oxidant (Fe3+, 02), surface electron transfer, and surface product formation and detachment. The overall oxidation process, without considering the detailed mechanisms, is demonstrated below using pyrite (FeS2) (Evangelou, 1995b) ... 

The transition metal trichalcogenides MX3 (M = Ti, Zr, Hf, Nb, Ta X = S, Se) have been studied in some detail. All the trichalcogenides contain anion-anion bonding and consequently a redox competition exists between electron transfer to the metal center and electron transfer to the anions. The situation is the converse of the deintercalation of the thallium vanadium sulfides, where the competition is between oxidation of T1+ cations and that of the metal sulfur framework. 

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