Selenium and Sulfur

The acyclic and macromonocyclic polythioethers were well illustrated in CCC (1987 Section 47.8.3.2).1 Only a few recent examples will be given of this chemistry here. 

Thiolates are strong ligands and a rich coordination chemistry with cobalt(III) exists. Normally, thiols are employed directly in synthesis, and this is the usual entry into their coordination chemistry. However, alternative approaches do appear from time to time, and recently the cobalt-mediated direct thiolation of an aromatic ring in bidentate phenyl(2-pyridyl)diazene using dithiocarbamate ion to produce a coordinated tridentate thiolate has been reported.1034 

Chelation of ixo-maleonitriledithiolate (imdt) has been structurally characterized in the octahedral cobalt(III) complex trmw-[Co(imdt)2(P(ra-Bu)3)2], formed via reaction of cobalt(II) ion with K2(imdt) in the presence of the phosphine.1037 Simple chelating thiolates such as SCH2CH2S not only form mononuclear compounds, but participate in bridging in clusters such as [Co3(SCH2CH2S)3(PEt3)3]3+ (243).1038 

When electrochemical oxidation of Co metal is carried out in an acetonitrile solution of phosphinothiol ligands, tris(phosphinothiolate)cobalt(III) compounds are formed.1039 The octahedral P,S-chelated Co(2-(Ph2P)-6-(Me3Si)C6H3S)3 compound has been characterized structurally. 

Tri-tert-butoxidesilanethiolate forms complexes with both Co11 and Co111. Reaction of the thiol with a Co11 ammine in water yields a dimer which undergoes oxidation in an ammonia-saturated atmosphere to form octahedral [Co(SSi(0-t-Bu)3)2(NH3)4]+, the first silanethiolate characterized structurally.1044 

Vibrational frequencies of diatomic S2 and 82 and triatomic S3, 83 , and 83 are listed in Secs. 2.1 and 2.2, respectively. The IR spectra of the 84 molecule produced in Ar matrices suggest the formation of two open-chain isomers [ 1745]. 84 vapor consists of green-absorbing (k ax, 530 nm) and red-absorbing (560-660 nm) molecules. Raman spectra show that the former is a chain-like rra z -planar molecule of C2h symmetry whereas the latter is a branched-ring molecule [1746]. 

As is shown in Fig. 2.77 larger cluster molecules such as 85 [1747], 87 [1748], 8s [1749], 89 [1750], and 812 [1749] take puckered ring structures and their vibrational spectra have been reported. In general, the neutral molecules 8 (n = 6-12) take ring structures, while the anions 8 (n = 4-6) take open-chain structures. 

The 8 ion assumes a nonplanar C2 structure similar to that of H2O2 (dihedral angle, 98°), and its Raman spectrum has been assigned [1751]. In (NH4)285, the 85 ion assumes a helical chain of C2 symmetry, while in Na28s, it takes a cis conformation of Cs symmetry. The Raman spectrum of the former has been assigned using normal coordinate analysis [1752]. The influence of the cation on the structures and spectra of these anions have been investigated [1753]. 

As is shown in Fig. 2.78, the SgO molecule is an eight-membered crown-shaped Sg ring to which an oxygen atom is bonded (Q symmetry). Steudel and coworkers have measured the IR/Raman spectra of SgO and made complete assignments based on normal coordinate analysis [1757,1758]. These workers also reported the Raman spectra of S9O and S iqO, which may acquire similar ring structures [ 1750]. The Raman 

Low-temperature Raman spectra of SCI2, SCI4, and S2CI2 have been reported [1775]. The 8214 ion assumes a butterfly structure of C2 (AsF6 salt) or C21 (SbF salt) symmetry, and the v(S-S) and v(I-I) vibrations are at 734 and 227 cm respectively. The IR/Raman spectra of the ion (X a halogen) containing the S7 ring are 

N2O is currently the third of the greenhouse gases , after carbon dioxide and methane in importance. While not as abundant as carbon dioxide, it is 300 times more potent in its ability to warm the planet. 

The oxidation state of sulfur is +6 in sulfate and —2 in H2S or RSH. An interesting feature of sulfur chemistry is that the amino acid cysteine, as its RS (cysteinate) form, acts as a base, binding H+, but also a number of transition metals, including Fe, Zn, Mo, and Cu. 

Hydrogen sulfide can be oxidised to elemental sulfur, for example, by green and purple sulfur bacteria. Further oxidation of elemental sulfur by sulfur oxidising bacteria can produce sulfate. 

He gave it the name selenium after the moon goddess Selene. Tellurium, named after the Roman god of the earth, had been isolated a few years earlier. 

In summary, the codon TGA, which normally codes for termination of protein synthesis is programmed from a distance to encode the 21st amino acid selenocysteine a special tRNA is loaded in a unique and unorthodox way, incorporating a selenocysteine residue which is synthesised de novo at the tRNA level by special enzymes highly specialised proteins are required to recognise and bind the secondary mRNA structures and the tRNA specialised elongation factors have to compete with canonical ones and with release factors. No wonder that chemists and biochemists ask what are the unique properties of selenocysteine compared to cysteine (Amer, 2010) to justify the involvement of so many molecular partners (Allmang, Wurth, Krol, 2009). 

Several forms of selenium are known also. In addition to two red (monoclinic) and a grey (hexagonal) form (with melting point 217° C), there have been reported several forms of amorphous selenium (both red and black). The grey form (and presumably also the amorphous forms) of selenium consist of zig-zag chains of bound selenium atoms, but one of the red forms has been shown to contain Se8 rings. The grey form of selenium has become especially important in recent years the sharp increase in its electrical conductivity upon exposure to light makes it a valuable material for use in photoconductive cells. 

The action of strong base on elemental sulfur or selenium is formally similar to the basic hj drolysis of the halogens and of phosphorus  

As was the case for the basic hydrolysis of phosphorus, tracing the path of basic hydrolysis of the chalcogens is a formidable task the reactions are two-phase reactions and proceed through a series of steps of which the first step is probably the slowest (making the isolation of intermediates difficult). Note that as with hydrolyses of the halogens, the reaction is reversible—that is, that acidification of a solution containing sulfide and sulfite (or selenide and selenite) yields the elementary chalcogen. 

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The first compounds with a Aj selenazoline structure were mentioned in the literature in 1892 by Michels (55). He prepared 2-methyl-A -selenazoline in the course of his investigations into sulfur and selenium... 

Comparison of the ultraviolet spectra of analogous sulfur and selenium compounds shows that there is very little difference in the absorption curves, except for a slight bathochromic shift in the case of the selenium derivatives-... 

X in acid derivatives Name of X (in priority order fluoride, chloride, bromide, iodide, cyanide, azide then the sulfur and selenium analogs)... 

In Group 15 (V), nitrogen compounds readily form molecular compounds with BF. Phosphoms compounds also form adducts with BF. Inorganic or organic compounds containing oxygen form many adducts with boron trifluoride, whereas sulfur and selenium have been reported to form only a few (41—43). 

Replacement of Boron by Sulfur and Selenium. Trialkylboranes are cleaved by dialkyl- and diaryldisulfides in an air-catalyzed radical reaction producing mixed thioethers (259). 

Although tellurium resembles sulfur and selenium chemically, it is more basic, more metallic, and more strongly amphoteric. Its behavior as an anion or a cation depends on the medium, eg ... 

Tellurium forms inorganic compounds very similar to those of sulfur and selenium. The most important teUurium compounds are the teUurides, haUdes, oxides, and oxyacids (5). Techniques and methods of preparation are given in the Uterature (51,52). The chemical relations of teUurium compounds are iUustrated in Figure 2 (53). 

Halogen, Sulfur, and Selenium Group Transfer Reactions... 

The UV absorption spectrum of benzoisotellurazole is similar to those of its sulfur and selenium analogues, all absorption bands of the former being bathochromi-cally shifted relative to the latter compounds. The NMR signal of the H-3 proton... 

The molecular and crystal stmctures of 2-phenylbenzotellurazole were determined by X-ray (89KGS1690). The dihedral angle between the planes of the five-membered heterocycle and 2-phenyl ring is 31.2°. In contrast to benzoisotel-lurazole, no shortened intermolecular Te- N contacts were found in the crystal stmcture of 2-phenylbenzotellurazole. Consequently, no anomalies in solubility and melting point were revealed for this compound as compared with its sulfur and selenium congeners. 

Sulfur and selenium analogues of 91 are also susceptible of this type of rearrangement. Its rate is defined by the strength of the C-chalcogen bonds in 91 and analogues and was found to increase in the order of S < Se < Te (94MI1). 

Mass spectra of l,6-dioxa-2,5-diaza-6<2-tellurapentalenes are given in (75JHC639). The ESCA spectra of the parent heterocycle and of its sulfur and selenium analogues were discussed (80JA1783). 

These trends are general ones, observed with other oxoadds of the nonmetals. Recall, for example, that nitric acid, HNO3 (oxid. no. N = +5), is a strong acid, completely ionized in water. In contrast, nitrous add, HN02 (oxid. no. N = +3), is a weak acid (Ka = 6.0 X 10-4). The electronegativity effect shows up with the strengths of the oxoadds of sulfur and selenium ... 

Very little is known about chalcogenide halides of Group IVB elements. Although the existence of sulfide chlorides (45, 274, 329, 365) and of a selenide chloride (329) of titanium was claimed in early publications, their true composition, and even their existence, remains doubtful. They have usually been obtained by the reaction of titanium chlorides with sulfur and selenium, respectively, or with hydrogen sulfide. The synthesis of a pure compound, TiSClj, was published in 1959 (113). It is an intermediate of the reaction of TiCU with HjS. 

The elements sulfur and selenium, which combine with the hydrogen evolved to give, respectively, H2S and H2Se. Little is known about this mechanism either. ... 

Complexes of Mercury(l) with Sulfur- and Selenium-Donor Ligands. 

This ch ter contains reactions which prepare the oxides of nitrogen, sulfur and selenium. Included are W-oxides, nitroso and nitro compounds, nitrile oxides, sulfoxides, selenoxides and sulfones. Oximes are considered to be amines and appear in those sections. Preparation of sulfonic acid derivatives are found in Chapter Two and the preparation of sulfonates in Chapter Ten. 

A method to circumvent the problem of chalcogen excess in the solid is to employ low oxidation state precursors in solution, so that the above collateral reactions will not be in favor thermodynamically. Complexation strategies have been used for this purpose [1, 2]. The most established procedure utilizes thiosulfate or selenosulfate ions in aqueous alkaline solutions, as sulfur and selenium precursors, respectively (there is no analogue telluro-complex). The mechanism of deposition in such solutions has been demonstrated primarily from the viewpoint of chemical rather than electrochemical processes (see Sect. 3.3.1). Facts about the (electro)chemistry of thiosulfate will be addressed in following sections for sulfide compounds (mainly CdS). Well documented is the specific redox and solution chemistry involved in the formulation of selenosulfate plating baths and related deposition results [11, 12]. It is convenient to consider some elements of this chemistry in the present section. 

Patrone, L.. Palacin, S. and Bourgoin, J.P. (2003) Direct comparison of the electronic coupling efficiency of sulfur and selenium alligator dips for molecules adsorbed onto gold electrodes. Applied Surface Science, 212—213, 446—451. 

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