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144 Ingo Krossing unstable in the gas phase with respect to two S2·+ ions, is lattice stabilized in the solid state by the much higher lattice potential enthalpies of a 2:1 vs a 1:1 salt [11]. For example, solid S4(AsF6)2 is by 362 kJ mol1 favored over 2 S2(AsF6). In order to overcompensate the lattice enthalpy gain of S42+ salts and stabilize a S2·+ salt, anions with a thermochemical volume of at least 5000 3 would be needed [11]. Given the thermochemical volume of the AsF6 anion of 110 3 this appears unlikely to be achieved in the future! 6.3 Dissociation of Sn2+ (n=4, 6, 8, 10) in the Gas Phase and in Solution The S82+ dication is also lattice stabilized in the solid state [1a, 23, 24]. In 1994 [24] it was shown that isolated gaseous S82+ is unstable with respect to a stoichiometric dissociation into various monocations Sn·+ (n=2–7) and by analogy one expects the same to hold for S192+. However, due to the great difficulties in obtaining a good quantum chemical calculation of the gaseous S82+ cation, it was not before the year 2000 that reliable numbers could be put on the various dissociation equilibria [23]. Moreover, the puzzling fact that solutions of Sn2+ (n=8, 19) salts in several solvents also contain varying amounts of sulfur radical cations Sn+ (n4) sparked additional quantum chemical calculations which included approximate solvation energies [3] as calculated by one of the modern PCM models (=Polarizable Continuum Model) developed by Thomasi et al. [25]. Other recent publications on the use of PCM solvation models established the quality of this method [26]. The results of these calculations are collected in Table 2. From the energetics summarized in Table 2 it is evident that all doubly charged gaseous sulfur cations are unstable with respect to a dissociation into monocations due to a relief of electrostatic repulsion upon dissociation (i.e., a Coulomb explosion). However, the solvation energies of dications are much higher than those of monocations and, consistently, none of the disso- ciation reactions in Table 2 giving only monocations is exergonic in solu- tion. But how are the monocations in solutions of S82+ and S192+ salts then formed? Especially bearing in mind that both observed or tentatively as- signed monocations S5·+ and S7·+ have a lower average oxidation state than S82+. This then pointed out that in dissociation reactions of S82+ another spe- cies with a higher average oxidation state than S82+ must form. S42+ could be a player; however, it was convincingly shown that in the absence of halogen facilitator the S42+ oxidation stage was never reached [9] for kinetic reasons and therefore this dication, although thermochemically possible, is no player in equilibria of dissolved S82+. Eventually and in agreement with a reinter- pretation of earlier experimental work it was concluded that the higher oxi- dized species necessary to account for the observation of reduced S5·+ and S7·+ is the D3d symmetric S62+ dication (=dimer of S3·+). The S62+ dication with 10p electrons shown in Fig. 5 is apparently also responsible for the in- tense blue color of solutions of S82+ salts (see below) and the most favored dissociation reaction of S82+, excluding S42+, is Eq. (g) in Table 2. The picture changes when a small amount of halogen facilitator is added: the formationPDF Image | Topics in Current Chemistry
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