The Polysulfide Ions

The color of the alkali polysulfides changes with increasing sulfur content from the light yellow of disulfides through orange to red or red-brown. Hexasulfides liberate sulfur when dissolved in water, which is understandable from the measurements of Amtson et al. (8) of the solubility of orthorhombic sulfur in aqueous sodium monosulfide. The equilibrium value of n in S was found to be approximately 4.8 at 25°, and varied at 50° from approximately 4.6 in dilute solutions to approximately 5.4 at higher concentrations. 

The di- and trisulfide ions offer no structural problem apart from bond lengths, and the bond angle in the trisulfide. For the tetrasulfide and higher polysulfides branched structures can be imagined, such as a tetrathio-sulfate structure for the pentasulfide. The recent crystal structure deter- 

The crystals of barium tetrasulfide monohydrate, BaS4 H20, are orthorhombic, with four formula units in a cell of dimensions a = 9.67 A, b = 7.99 A, c = 7.81 A. The space group is Dt -P212l2. The unbranched, nonplanar tetrasulfide ions lie on twofold axes of symmetry, with only right-handed or only left-handed forms in each unit cell. 

Cesium hexasulfide, Cs e, crystallizes in the triclinic space group C7-P1 with a = 11.53 A, b = 9.18 A, c = 4.67 A, a = 89.2°, 0 = 95.2°, 7 = 95.1°, and two formula units per unit cell. The hexasulfide ions are unbranched and nonplanar and have the shapes of extended helixes. 

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The sulfur analog of hydrogen peroxide also exists and is an example of a polysulfane, a catenated molecular compound of composition HS—S —SH, where n can take on values from 0 through 6. The polysulfide ions obtained from the polysulfanes include two ions found in lapis lazuli (Fig. 15.15). 

The latter reaction has been studied numerous times because of its relevance for the autoxidation of hydrogen sulfide in seawater and other aqueous systems [112, 113]. 8ince the polysulfide ions can be further oxidized to elemental sulfur which precipitates from the solution, these reactions are the basis for several industrially important desulfurization processes (e.g., the 8tretford, 8ulfolin, Lo-Cat, 8ulFerox, and Bio-8R processes) [114] ... 

Polysulfides of several metals can be prepared by reaction of the metals with excess sulfur in liquid NH3 (group IA metals) or by heating sulfur with the molten metal sulfide. The polysulfide ion binds to metals to form coordination compounds in which it is attached to the metal by both sulfur atoms (as a so-called bidentate ligand). One example is an unusual titanium complex containing the S52-ion that is produced by the following reaction (the use of h to denote the bonding mode of the cyclo-pentadienyl ion is explained in Chapter 16) ... 

Carbon-sulfur hybrid materials, i.e., porous carbons [58,59] or CNTs having nanosized S in the pores or channels, are the most promising solution for the Li-S battery to increase the electronic and ionic conductivity of sulfur or sulfide, and prevent, to a great extent, the solubility of the polysulfide ions formed on reduction of S or upon oxidation of insoluble sulfides [60]. An intimate contact between carbon and sulfur is essential [61]. 

Obviously, the donor activity of the nucleophile, that is, the sulfide ion is enhanced as the negative charge is dispersed along the polysulfide ion produced from the sulfide on the addition of elemental sulfur. This increases the mobility of electrons and facilitates electron transfer. That is why this reaction can be initiated in such a simple way as the addition of elemental sulfur. 

Bacterially produced elemental sulfur can also react with hydrogen sulfide form polysulnde ions. Thus, polysulfide ions should constitute a significant fraction of sulfur nudeophiles in reducing sediments especially where sulfide oxidation is incomplete, such as in intertidal and salt marsh sediments (31321. The polysulfide ions should also be important at redox boundaries (anoxic/ suboxic) in the water column of marine anoxic basins, such as the Black Sea. 

The results obtained for reaction of acrylic acid (200 /iM) with tetrasulfide (1.25 mM 5 mM) as well as bisulfide (5 mM) are shown in Figurel. In the reaction with HS, the concentration of 3-MPA did not noticeably increase after treatment with TBP. If polysulfide ion was the actual reactive species in this reaction, then TBP treatment should have caused a significant increase of 3-MPA, in comparison to that without TBP. Therefore, these results indicate that HS was the reactive species in the sulfide medium. However, in the reaction of acrylate with the polysulfide ion, S42, determination of 3-MPA concentration after TBP treatment revealed a large increase, indicating that S42 reacted with acrylate to produce 3-tetrasulfidopropionic acid. The reactivity of tetrasulfide with acrylate was much higher than that of bisulfide. Similar results were also obtained for reaction of acrylonitrile with HS and S42 (Figure 2). The kinetic data are summarized in Table I. In the polysulfide senes low concentrations of 3-MPA were also observed without TBP treatment. It is possible that this 3-MPA was formed from uncatalyzed dissociation of the polysulfide addition product... 

In the later work (110) they reported the relative rates of additions of both bisulfide (HS") and polysulfide (S42 ) to acrylic acid and to acrylonitrile (CH2=CH-CN) for a range in pH and ionic strength. Results showed that at equal nucleophile concentrations, the addition of the polysulfide ion was much faster than that of the bisulfide ion. The difference in rate for the two nucleophiles was greater for acrylic acid, which is largely the anion under the pH conditions, than for acrylonitrile which is neutral. Effects of ionic strength... 

Polysulfates, most typically thiosulfates, are found in anoxic riverine sediments. Additionally inorganic polysulfides H2S (n = 1 -5) can be present even in oxygen-rich aquatic systems. The polysulfide ions can readily be converted to OCS or perhaps carbon disulfide under sunlight irradiation (Gun et al., 2000) or via ... 

The reaction of polysulfides with peroxide depends on the polysulfide ion present (see above). Once higher polysulfides are produced, the reaction should result in a peroxide-polysulfide intermediate (similar to II) that transfers two electrons from the polysulfide ion to the peroxide as readily as the sulfide and peroxide reaction. At low peroxide levels, partial oxidation of the polysulfide ions should result in the direct formation of sulfate (through thiosulfate and perhaps sulfite) and S ... 

The only instances in which sulfur forms only one bond to sulfur and none to another atom, are apparently in the polysulfide ions as terminal atoms, and in the thiosulfate and thiosulfonate ions, S—S03 and... 

During charging, bromide ions are oxidized to tribromide ions, and they emit electrons at the positive electrode, while the polysulfide ions are reduced at the negative electrode. To maintain electrical balance within the electrolytes, sodium ions usually transfer from anolyte to catholyte. In a discharging process, reverse reactions occur. 

The dinuclear ion Mo2(S2) g (F - prepared from the reaction of molybdate and polysulfide solution (13) is a usehil starting material for the preparation of dinuclear sulfur complexes. These disulfide ligands are reactive toward replacement or reduction to give complexes containing the Mo2S " 4 core (Fig. 3f). 

Zinin Reduction. The method of reducing aromatic nitro compounds with divalent sulfur is known as the Zinin reduction (57). This reaction can be carried out in a basic media using sulfides, polysulfides, or hydrosulfides as the reducing agent. These reactions can be represented as follows when the counter ion is sodium ... 

Barium sulfide solutions undergo slow oxidation in air, forming elemental sulfur and a family of oxidized sulfur species including the sulfite, thiosulfate, polythionates, and sulfate. The elemental sulfur is retained in the dissolved bquor in the form of polysulfide ions, which are responsible for the yellow color of most BaS solutions. Some of the mote highly oxidized sulfur species also enter the solution. Sulfur compound formation should be minimized to prevent the compounds made from BaS, such as barium carbonate, from becoming contaminated with sulfur. 

NMR and visible spectra have established that a number of S-N anions are present in such solutions.The primary reduction products are polysulfides Sx, which dissociate to polysulfur radical anions, especially the deep blue 83 ion (/Imax 620nm). In a IM solution the major S-N anion detected by NMR spectroscopy is cycZo-[S7N] with smaller amounts of the [SSNSS] ion and a trace of [SSNS]. The formation of the acyclic anion 5.23 from the decomposition of cyclo-Sjl is well established from chemical investigations (Section 5.4.3). The acyclic anions 5.22 and 5.23 have been detected by their characteristic visible and Raman spectra. It has also been suggested that a Raman band at 858 cm and a visible absorption band at 390 nm may be attributed to the [SaN] anion formed by cleavage of a S-S bond in [SSNS]. ° However, this anion cannot be obtained as a stable species when [SsN] is treated with one equivalent of PPhs. 

Therefore, polysulfide ions play a major role in the global geological and biological sulfur cycles [1, 2]. In addition, they are reagents in important industrial processes, e.g., in desulfurization and paper production plants. It should be pointed out however that only sulfide, elemental sulfur and sulfate are thermodynamically stable under ambient conditions in the presence of water, their particular stabihty region depending on the redox potential and the pH value [3] ... 

The generated polysulfide dianions of different chain-lengths then establish a complex equilibrium mixture with all members up to the octasulfide at least see Eqs. (5) and (6). For this reason, it is not possible to separate the polysulfide dianions by ion chromatography [6]. The maximum possible chain-length can be estimated from the preparation of salts with these anions in various solvents (see above). However, since the reactions at Eqs. (22) and (23) are reversible and Sg precipitates from such solutions if the pH is lowered below a value of 6, the nonasulfide ion must be present also to generate the Sg molecules by the reverse of the reaction at Eq. (22). The latter reaction (precipitation of Sg on acidification) may be used for the gravimetric determination of polysulfides [11]. There is no evidence for the presence of monoprotonated polysulfide ions HS - in aqueous solutions [67, 72]. 

The composition of sodium polysulfide solutions saturated with sulfur of zero oxidation number (S°) has also been studied at 25 and 80 °C (solutions in contact with elemental sulfur) [76]. In this case the ratio 8° 8 per polysulfide ion increases with increasing alkahnity. The maximum average number of sulfur atoms per polysulfide molecule was obtained as 5.4 at 25 °C and 6.0 at 80 °C and pH values of >12. Equilibrium constants for reactions as in Eqs. (26) and (27) have been derived assuming various models with differing numbers of polysulfide ions present. 

Under special conditions the rapid protonation of aqueous polysulfide ions by hydrochloric acid at temperatures of below 0 °C produces polysulfanes ... 

In a similar fashion, cyanide ions desulfurize polysulfide ions at 100 °C to the monosulfide level, e.g. ... 

The molecular structures of the isolated polysulfide monoanions 8 with n=2-9 have been studied by density functional calculations and those of the smaller ions also by ab initio MO calculations. Compared to the neutral 8 molecules the extra electron occupies an antibonding orbital resulting in longer 88 bonds. The species 83 is bent and of C2V symmetry (a=115°) [140, 141]. 84 was calculated to be a planar ion of C2V symmetry (similar to the neutral molecule 84) but the planar C2h structure is only slightly less stable [140, 141]. The most stable isomer of 85 is a chain of Q symmetry sim-... 

An alternative ambient temperature design based on sodium ion reaction refers to the domain of the so-calledbatteries. The polysulfide bromide cell (PSB) provides a reversible electrochemical reaction between two salt solution electrolytes (sodium bromide and sodium polysulfide), according to the scheme... 

Sulfide ions in solution react with sulfur to produce polyatomic anions known as the polysulfides. When... 

The phase Na2Sx is sodium polysulfide, a material with a sulfur content of between 3 and 5. The anode reaction takes place at the liquid sodium - (3"-alununa interface. Here sodium atoms lose an electron and the Na+ ions formed enter the conduction planes in the electrolyte. The cathode reaction, which occurs at the interface between the (3"-alumina and the liquid sulfur forms sodium polysulfides. Despite the desirable properties of the cell, technical and economic considerations have acted so as to curtail large-scale commercial production. 

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