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Homoatomic Sulfur Cations 143 Table 1 Enthalpies of formation of gaseous and solid sulfur cations in kJ mol1 at 298 K Cation S2·+ DfH298(g) 1031 [15] Dication – DfH298(g) – Salt – DfH298(s) – S3·+ 1076 [15] S4·+ 972 [21] S5·+ S6·+ S7·+ S8·+ – S32+ S42+ 2538 [3] 2318 [21] – S4(AsF6)2a – 3104 [11] – S62+ – 2238 [3] – S6(AsF6)2 – 3103 [3] – S82+ S102+ – 2151 [23] 2146 [3] – S8(AsF6)2 – – 3122 [23] – 939 971 [15] [15] [15] 951 976 [15] – a S4(AsF6)2(AsF3): 4050 [11] of S102+ shows a similar bond length alternation as observed in S192+ with similarly long S-S distances around the two tricoordinate sulfur atoms. 6 Thermodynamics of the Sulfur Cations 6.1 Basic Thermochemical Data The enthalpies of formation of several gaseous and solid sulfur cations were experimentally determined or computationally assessed. Table 1 summarizes the currently established “best” values. 6.2 The Preference of S42+ Over 2 S2•+ in the Solid State In order to understand the observation that oxygen in the oxidation state +0.5 forms salts that contain the O2·+ cation with a variety of anions while sulfur in the same oxidation state forms a dimer of the composition S42+[=(S2·+)2], the gas phase dissociation enthalpy of S42+ giving two S2·+ was assessed by quantum chemical calculations [20, 21], i.e. [(g)=gaseous]: S2þ ! 2Sþ DrHðgÞ 1⁄4 ? ð4Þ 4ðgÞ 2ðgÞ Initial results gave dissociation enthalpies that ranged from 200 to 530 kJ mol1 with a rather erratic behavior within all methods employed [20]. A systematic investigation of correlation and basis set effects on this reaction enthalpy finally revealed a dissociation enthalpy of 257 kJ mol1 obtained at the extremely expensive CCSD(T)/cc-pV5Z level of theory, while the B3PW91/6–311+G(3df) level was the only affordable energy calculation that systematically gave the same result [20]. Using this reaction enthalpy, a suitable Born-Haber cycle and estimation of the lattice potential enthalpies of the respective salts eventually showed that the preparation of solid S2·+ salts is impossible with all currently available anions and that S42+, althoughPDF Image | Topics in Current Chemistry
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