Bonding, Structure, and Energetics of Gaseous E82+ and of Solid E8(AsF6)2 (E = S, Se)

The attempt to prep. hitherto unknown homopolyat. cations of sulfur by the reaction of elemental sulfur with blue S8(AsF6)2 in liq. SO2/SO2ClF, led to red (in transmitted light) crystals identified crystallog. as S8(AsF6)2. The X-ray structure of this salt was redetd. with improved resoln. and cor. for librational motion: monoclinic, space group P21/c (No. 14), Z = 8, a = 14.986(2) .ANG., b = 13.396(2) .ANG., c = 16.351(2) .ANG., b = 108.12(1) Deg. The gas phase structures of E82+ and neutral E8 (E = S, Se) were examd. by ab initio methods (B3PW91, MPW1PW91) leading to DfH0[S82+, g] = 2151 kJ/mol and DfH0[Se82+, g] = 2071 kJ/mol. The obsd. solid state structures of S82+ and Se82+ with the unusually long transannular bonds of 2.8-2.9 .ANG. were reproduced computationally for the first time, and the E82+ dications were shown to be unstable toward all stoichiometrically possible dissocn. products En+ and/or E42+ [n = 2-7, exothermic by 21-207 kJ/mol (E = S), 6-151 kJ/mol (E = Se)]. Lattice potential energies of the hexafluoroarsenate salts of the latter cations were estd. showing that S8(AsF6)2 [Se8(AsF6)2] is lattice stabilized in the solid state relative to the corresponding AsF6- salts of the stoichiometrically possible dissocn. products by at least 116 [204] kJ/mol. The fluoride ion affinity of AsF5(g) was calcd. to be 430.5 +- 5.5 kJ/mol [av. B3PW91 and MPW1PW91 with the 6-311+G(3df) basis set]. The exptl. and calcd. FT-Raman spectra of E8(AsF6)2 are in good agreement and show the presence of a cross ring vibration with an exptl. (calcd., scaled) stretching frequency of 282 (292) cm-1 for S82+ and 130 (133) cm-1 for Se82+. An atoms-in-mols. anal. (AIM) of E82+ (E = S, Se) gave eight bond crit. points between ring atoms and a ninth transannular (E3-E7) bond crit. point, as well as three ring and one cage crit. points. The cage bonding was supported by a natural bond orbital (NBO) anal. which showed, in addn. to the E8 s-bonded framework, weak p bonding around the ring as well as numerous other weak interactions, the strongest of which is the weak transannular E3-E7 [2.86 .ANG. (S82+), 2.91 .ANG. (Se82+)] bond. The pos. charge is delocalized over all atoms, decreasing the Coulombic repulsion between pos. charged atoms relative to that in the less stable S8-like exo-exo E82+ isomer. The overall geometry was accounted for by the Wade-Mingos rules, further supporting the case for cage bonding. The bonding in Te82+ is similar, but with a stronger transannular E3-E7 (E = Te) bonding. The bonding in E82+ (E = S, Se, Te) can also be understood in terms of a s-bonded E8 framework with addnl. bonding and charge delocalization occurring by a combination of transannular np*-np* (n = 3, 4, 5), and np2 -> ns* bonding. The classically bonded S82+ (Se82+) dication contg. a short transannular S+-S+ (Se+-Se+) bond of 2.20 (2.57) .ANG. is 29 (6) kJ/mol higher in energy than the obsd. structure in which the pos. charge is delocalized over all eight chalcogen atoms. [on SciFinder (R)]