Anionic polysulfides

The pyrites and marcasite structures can be thought of as containing 82 units though the variability of the interatomic distance and other properties suggest substantial deviation from a purely ionic description. Numerous higher polysulfides S have been characterized, particularly for the more electropositive elements Na, K, Ba, etc. They are yellow at room temperature, turn dark red on being heated, and may be thought of as salts of the polysulfanes 

Structural Inorganic Chemistry, 5th edn.. Chap. 17 pp. 748-87, Oxford University Press, 1984. 

Dance and K. Fisher, Prog. Inorg. Chem. 41, 637-803 (1994). A comprehensive review with 503 references, 100 structural diagrams and 40 pages of tabulated material. 

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Obviously, in solution, 83 is not stable against oxidation. It is stable in the mineral lapis lazuli, and the industrial ultramarine blue pigment [28]. In these materials, the radical 83 is encapsulated in the -cages of the sodalite structure, which protects it against oxidation. In ultramarine pigments, another radical anion polysulfide, 82 , has been observed. 

It is supposed to originate from the dissociation of 84 . This dissociation is perhaps significant at high temperatures, like those of the synthesis of ultramarine pigments or those of doping of alkali halide crystals. 83 and 82 radicals have also been observed in the alkali halides doped with sulfur [30, 31]. Another radical anion polysulfide, 84 , has been identified, by EPR experiments, in solution in DMF, originating from the dissociation of 8g , which is the least reduced polysulfide in this solvent [32]. 

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. 

The red [SSNO] anion (9.2) (2max 448 nm) is produced by the reaction of an ionic nitrite with elemental sulfur or a polysulfide in acetone, DME or DMSO. ° The formation of 9.2 probably proceeds via an intermediate such as the [S2NO2] anion. This process is thought to occur in the gunpowder reaction, which also entails the reaction of potassium nitrite (produced by reduction of potassium nitrate with charcoal) and sulfur. 

C2h isomer (8) is less well characterized but is said to result from the reaction of hexachlorobutadiene, CCI2 =CC1—CCl=CCl2, with polysulfide anions. The treatment of S2CI2 with [NBu4]2[Zn(o -C3S5)2] yields a mixture of C3Sg and C6S12 which can be separated by fractional crystallization from CS2 ... 

A rather different series of cyclic thiophos-phate(III) anions [(PS2) ]" is emerging from a study of the reaction of elemental phosphorus with polysulfidic sulfur. Anhydrous compounds... 

Sulfur is known to be easily reducible in nonaqueous solvents and its reduction products exist at various levels of reduction of polysulfide radical anions (S . ) and dianions (Sm2 ) 173], Recently Be-senhard and co-workers [74] have examined the effect of the addition of polysulfide to LiC104-PC. Lithium is cycled on an Ni substrate with Qc=2.7 C cm 2 and cycling currents of 1 mA cm. The cycling efficiency in PC with polysulfide is higher than that without an additive. The lithium deposition morphology is compact and smooth in PC with added polysulfide, whereas it is dendritic in PC alone. 

Batsanov et al. 23) reacted sulfur with PtCU and PtBr2 by heating mixtures of the reactants in evacuated, sealed ampoules. At 100 -200°C after 12-24 h, sulfide chlorides PtCljS (1.70 < x < 2 0.6 s y < 3.35) and sulfide bromides PtBr S (1.87 < x 2.06 0.84 y s 1.80) were formed. The compositions depended on the initial PtX2 S ratio, and the temperature. At 320-350°C, loss of chlorine led to the compounds PtClS (1.7 y 1.9). According to their X-ray powder patterns, all of these products retained the main structural features of the original platinum halides. From considerations of molar volumes, the authors deduced the presence of polysulfide anions. 

Since the chain-lengths of the molecules present in crude sulfane oil is different from the chain-length of the anions in the original sodium polysulfide solution one has to conclude that in addition to the reaction at Eq. (4) the reactions at Eqs. (5) and (6) also take place during the preparation by protonation of the polysulfide anions. 

Interestingly, the sulfanes H2S are both proton acceptors and donors. In the first case sulfonium ions H3S are formed, in the second case hydrogen polysulfide anions HS are the result. While the latter have never been isolated in salts, several salts with sulfonium cations derived from the sulfanes with n = 1, 2, and 4 have been published. However, none of these salts has been structurally characterized by a diffraction technique. Therefore, the structures of the HsSn cations and HS anions are known from theoretical calculations only. 

Abstract Inorganic polysulfide anions and the related radical anions S play an important role in the redox reactions of elemental sulfur and therefore also in the geobio chemical sulfur cycle. This chapter describes the preparation of the solid polysulfides with up to eight sulfur atoms and univalent cations, as well as their solid state structures, vibrational spectra and their behavior in aqueous and non-aqueous solutions. In addition, the highly colored and reactive radical anions S with n = 2, 3, and 6 are discussed, some of which exist in equilibrium with the corresponding diamagnetic dianions. 

The chemistry of polysulfide dianions is closely related to that of the radical-monoanions S since both types of anions are in equilibrium with each other in solution and in high-temperature melts, e.g. ... 

Furthermore, polysulfide anions are subject to autoxidation if molecular oxygen is present, e.g. ... 

In solution this reaction is rather rapid but in the solid state autoxidation takes place much slower. Nevertheless, commercial sulfides and polysulfides of the alkali and alkali earth metals usually contain thiosulfate (and anions of other sulfur oxoacids) as impurities [6]. For all these reasons the chemistry of polysulfides is rather complex, and some of the earlier studies on polysulfides (prior to ca. 1960) are not very rehable experimentally and/or describe erroneous interpretations of the experimental results. 

Many of the polysulfides described above have been investigated by X-ray diffraction on either powders or single crystals. In all cases the more sulfur-rich anions (n>3) form unbranched chains the symmetry of which varies between Ci, C2, and Cs. According to Fig. 1 the symmetry C2 results if all torsion angles have the same sign (right-handed helix + + +... left-handed helix ----...). If the different torsion angles of the anion vary between + and... 

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]. 

If Sg is dissolved in a polysulfide solution the reactions according to Eqs. (24) and (25) are faster than the reactions at Eqs. (22) and (23) since disulfide (and also trisulfide) anions are evidently stronger nucleophiles than HS ... 

Consequently, sulfur dissolves in polysulfide solutions much faster than in equimolar monosulfide solutions [73]. In this context it is of interest that the analogous decaselenium dianion Scio has been prepared and structurally characterized in solid [PPN]2Seio [74]. This anion is however bi-cyclic. 

Therefore, the pH values of these solutions are between 11 and 12. The speciation model used by 8chwarzenbach and Fischer is certainly too simple but these authors have been the first to demonstrate the strong dependence of the polysulfide anion distribution on the alkalinity. According to Eqs. (26)-(28) higher pH values in dilute solutions will favor smaller anion sizes. 

Ionic polysulfides dissolve only in media of high polarity hke water, liquid ammonia, alcohols, nitriles, amines, and similar solvents. In all of these solvents 8 can be reduced electrochemically to polysulfide anions. On the other hand, the electrochemical oxidation of polysulfide anions produces elemental sulfur ... 

Ionic polysulfides dissolve in DMF, DMSO, and HMPA to give air-sensitive colored solutions. Chivers and Drummond [88] were the first to identify the blue 83 radical anion as the species responsible for the characteristic absorption at 620 nm of solutions of alkali polysulfides in HMPA and similar systems while numerous previous authors had proposed other anions or even neutral sulfur molecules (for a survey of these publications, see [88]). The blue radical anion is evidently formed by reactions according to Eqs. (5)-(8) since the composition of the dissolved sodium polysulfide could be varied between Na2S3 and NaaS with little impact on the visible absorption spectrum. On cooling the color of these solutions changes via green to yellow due to dimerization of the radicals which have been detected by magnetic measurements, ESR, UV-Vis, infrared and resonance Raman spectra [84, 86, 88, 89] see later. 

Vibrational spectroscopy and in particular Raman spectroscopy is by far the most useful spectroscopic technique to qualitatively characterize polysulfide samples. The fundamental vibrations of the polysulfide dianions with between 4 and 8 atoms have been calculated by Steudel and Schuster [96] using force constants derived partly from the vibrational spectra of NayS4 and (NH4)2Ss and partly from cydo-Sg. It turned out that not only species of differing molecular size but also rotational isomers like Ss of either Cy or Cs symmetry can be recognized from pronounced differences in their spectra. The latter two anions are present, for instance, in NaySg (Cs) and KySg (Cy), respectively (see Table 2). 

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