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

Abstract

The attempt to prepare hitherto unknown homopolyatomic cations of sulfur by the reaction of elemental sulfur with blue S8(AsF6)2 in liquid SO2/SO2ClF, led to red (in transmitted light) crystals identified crystallographically as S8(AsF6)2. The X-ray structure of this salt was redetermined with improved resolution and corrected for librational motion: monoclinic, space group P2(1)/c (No. 14), Z = 8, a = 14.986(2) A, b = 13.396(2) A, c = 16.351(2) A, beta = 108.12(1) degrees. The gas phase structures of E8(2+) and neutral E8 (E = S, Se) were examined by ab initio methods (B3PW91, MPW1PW91) leading to delta fH theta[S8(2+), g] = 2151 kJ/mol and delta fH theta[Se8(2+), g] = 2071 kJ/mol. The observed solid state structures of S8(2+) and Se8(2+) with the unusually long transannular bonds of 2.8-2.9 A were reproduced computationally for the first time, and the E8(2+) dications were shown to be unstable toward all stoichiometrically possible dissociation products En+ and/or E4(2+) [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 estimated 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 dissociation products by at least 116 [204] kJ/mol. The fluoride ion affinity of AsF5(g) was calculated to be 430.5 +/- 5.5 kJ/mol [average B3PW91 and MPW1PW91 with the 6-311 + G(3df) basis set]. The experimental and calculated FT-Raman spectra of E8(AsF6)2 are in good agreement and show the presence of a cross ring vibration with an experimental (calculated, scaled) stretching frequency of 282 (292) cm-1 for S8(2+) and 130 (133) cm-1 for Se8(2+). An atoms in molecules analysis (AIM) of E8(2+) (E = S, Se) gave eight bond critical points between ring atoms and a ninth transannular (E3-E7) bond critical point, as well as three ring and one cage critical points. The cage bonding was supported by a natural bond orbital (NBO) analysis which showed, in addition to the E8 sigma-bonded framework, weak pi bonding around the ring as well as numerous other weak interactions, the strongest of which is the weak transannular E3-E7 [2.86 A (S8(2+), 2.91 A (Se8(2+)] bond. The positive charge is delocalized over all atoms, decreasing the Coulombic repulsion between positively charged atoms relative to that in the less stable S8-like exo-exo E8(2+) isomer. The overall geometry was accounted for by the Wade-Mingos rules, further supporting the case for cage bonding. The bonding in Te8(2+) is similar, but with a stronger transannular E3-E7 (E = Te) bonding. The bonding in E8(2+) (E = S, Se, Te) can also be understood in terms of a sigma-bonded E8 framework with additional bonding and charge delocalization occurring by a combination of transannular n pi *-n pi * (n = 3, 4, 5), and np2-->n sigma * bonding. The classically bonded S8(2+) (Se8(2+) dication containing a short transannular S(+)-S+ (Se(+)-Se+) bond of 2.20 (2.57) A is 29 (6) kJ/mol higher i...

Isolation and reversible dimerization of a selenium–selenium three-electron σ-bond

“…This is due to the strong electrostatic repulsion of the two adjacent positive charges in À assuming ionic radii are similar, as shown by the Kapustiniskii equation (Fig. 6a) 41,[59][60][61] , where z x and z y are the charge of the ions, r x and r y the ionic radii and n is the number of ions per formula unit (for example, two for X þ Y À and three for X 2 þ (Y À ) 2 ). As shown in the Born-Fajans-Haber cycle (Fig.…”

The Nature of Transannular Interactions in E4N4 and E82+ (E = S, Se)

“…7,8 For S8 2+ , the high-resolution X-ray structure of the salt S8(AsF6)2 by Passmore et al is used as a reference, 19 while the optimized metrical parameters of Se8 2+ are compared with data for the dication in the salt (Te6 4+ )(Se8 2+ )(AsF6 -)6(SO2) as determined by Gillespie et al 17 In the crystallographic data, tetrachalcogen tetranitrides and octachalcogen dications have nearly ideal D2d and Cs molecular symmetries, respectively. 4,5,7,8,15,19 For E4N4, there are several short (3.1 -3.2 Å) E⋯N contacts between the four molecules in the unit cell. Similarly, short (2.7 -3.3 Å) E⋯F contacts exist between the chalcogen dications and AsF6 -anions in the X-ray data of both S8 2+ and Se8 2+ .…”

“…were subjected to Natural Bond Orbital (NBO), Atoms in Molecules (AIM) and An AIM analysis of the total electron densities of E8 2+ dications has previously been conducted by Passmore et al 19 The acquired data shows that a bond critical point is indeed found for the transannular E⋯E interaction and it is characterized by small values of both the electron density  (0.04 a.u. for S8 2+ ) and its Laplacian  2 (0.…”

“…The homopolyatomic chalcogen dications E8 2+ (4ac) (E = S, Se, Te) are also structurally related to the eight-membered chalcogennitrogen rings, but the presence of two more valence electrons compared to 1 gives them an overall chair-like molecular shape with only one weak cross-ring E⋯E contact. [15][16][17][18][19] The nature of transannular interactions in 14 has been a matter of much debate ever since the solid state structure of S4N4 was first reported. 14, A simple way to rationalize the cage-like structure of S4N4 using molecular orbitals from extended Hückel calculations was presented by Gleiter as early as in 1970.…”

Chemistry of Tetrasulfur Including Mysterious Rectangular S4 Metal Complexes

“…A colorless crystal of S 4 (AsF 6 ) 2 ¢AsF 3 was analyzed by X-ray crystallography. 33 The diamagnetic S 4 2+ possesses a square-planar structure (SS bond (¡): 1.999(4), 1.946(4), 1.924(4), and 1.986(4), SSS bond angle (deg): 91.7(2), 91.5(2), 87.9(2), and 88.9(2)), with a quasiaromatic 6³ four-membered ring. In the early stage of theoretical study of S 4 2+ , Fukui et al performed calculation of its electronic state using semiemprical INDO method for valence electrons and assigned the weak absorption band at 280 nm to the 2e u ¼ 3e u transition which is forbidden by symmetry.…”