RESUMO
The composition of a complex equilibrium mixture formed upon dissolution of (Se(6)I(2))[AsF(6)](2).2SO(2) in SO(2)(l) was studied by (77)Se NMR spectroscopy at -70 degrees C with both natural-abundance and enriched (77)Se-isotope samples (enrichment 92%). Both the natural-abundance and enriched NMR spectra showed the presence of previously known cations 1,4-Se(6)I(2)(2+), SeI(3)(+), 1,1,4,4-Se(4)I(4)(2+), Se(10)(2+), Se(8)(2+), and Se(4)(2+). The structure and bonding in 1,4-Se(6)I(2)(2+) and 1,1,4,4-Se(4)I(4)(2+) were explored using DFT calculations. It was shown that the observed Se-Se bond alternation and presence of thermodynamically stable 4ppi-4ppi Se-Se and 4ppi-5ppi Se-I bonds arise from positive charge delocalization from the formally positively charged tricoordinate Se(+). The (77)Se chemical shifts for cations were calculated using the relativistic zeroth-order regular approximation (ZORA). In addition, calculations adding a small number of explicit solvent molecules and an implicit conductor-like screening model were conducted to include the effect that solvent has on the chemical shifts. The calculations yielded reasonable agreement with experimental chemical shifts, and inclusion of solvent effects was shown to improve the agreement over vacuum values. The (77)Se NMR spectrum of the equilibrium solution showed 22 additional resonances. These were assigned on the basis of (77)Se-(77)Se correlation spectroscopy, selective irradiation experiments, and spectral simulation. By combining this information with the trends in the chemical shifts, with iodine, selenium, and charge balances, as well as with ZORA chemical shift predictions, these resonances were assigned to acyclic 1,1,2-Se(2)I(3)(+), 1,1,6,6-Se(6)I(4)(2+), and 1,1,6-Se(6)I(3)(+), as well as to cyclic Se(7)I(+) and (4-Se(7)I)(2)I(3+). A preliminary natural-abundance (77)Se NMR study of the soluble products of the reaction of (Se(4))[AsF(6)](2) and bromine in liquid SO(2) included resonances attributable to 1,1,4,4-Se(4)Br(4)(2+)(.) These assignments are supported by the agreement of the observed and calculated (77)Se chemical shifts. Resonances attributable to cyclic Se(7)Br(+) were also observed. The thermal stability of (Se(6)I(2))[AsF(6)](2).2SO(2)(s) was consistent with estimates of thermodynamic values obtained using volume-based thermodynamics (VBT) and the first application of the thermodynamic solvate difference rule for nonaqueous solvates. (Se(6)I(2))[AsF(6)](2).2SO(2)(s) is the first example of a SO(2) solvate for which the nonsolvated parent salt, (Se(6)I(2))[AsF(6)](2)(s), is not thermodynamically stable, disproportionating to Se(4)I(4)(AsF(6))(2)(s) and Se(8)(AsF(6))(2)(s) (DeltaG degrees for the disproportion reaction is estimated to be -17 +/- 15 kJ mol(-1) at 298 K from VBT theory).
RESUMO
The bonding in the highly homoatomic np pi-np pi (n > or = 3)-bonded S2I42+ (three sigma + two pi bonds), the Se-I pi-bonded Se2I42+ (four sigma + one pi bonds), and their higher-energy isomers have been studied using modern DFT and ab initio calculations and theoretical analysis methods: atoms in molecules (AIM), molecular orbital (MO), natural bond orbital (NBO), and valence bond (VB) analyses, giving their relative energies, theoretical bond orders, and atomic charges. The aim of this work was to seek theory-based answers to four main questions: (1) Are the previously proposed simple pi*-pi* bonding models valid for S2I42+ and Se2I42+? (2) What accounts for the difference in the structures of S2I42+ and Se2I42+? (3) Why are the classically bonded isolobal P2I4 and As2I4 structures not adopted? (4) Is the high experimentally observed S-S bond order supported by theoretical bond orders, and how does it relate to high bond orders between other heavier main group elements? The AIM analysis confirmed the high bond orders and established that the weak bonds observed in S2I42+ and Se2I42+ are real and the bonding in these cations is covalent in nature. The full MO analysis confirmed that S2I42+ contains three sigma and two pi bonds, that the positive charge is essentially equally distributed over all atoms, that the bonding between S2 and two I2+ units in S2I42+ is best described by two mutually perpendicular 4c2e pi*-pi* bonds, and that in Se2I42+, two SeI2+ moieties are joined by a 6c2e pi*-pi* bond, both in agreement with previously suggested models. The VB treatment provided a complementary approach to MO analysis and provided insight how the formation of the weak bonds affects the other bonds. The NBO analysis and the calculated AIM charges showed that the minimization of the electrostatic repulsion between EI2+ units (E = S, Se) and the delocalization of the positive charge are the main factors that explain why the nonclassical structures are favored for S2I42+ and Se2I42+. The difference in the structures of S2I42+ and Se2I42+ is related to the high strength of the S-S pi bond compared to the weak S-I sigma bond and the additional stabilization from increased delocalization of positive charge in the structure of S2I42+ compared to the structure of Se2I42+. The investigation of the E2X42+ series (E = S, Se, Te; X = Cl, Br, I) revealed that only S2I42+ adopts the highly np pi-np pi (n > or = 3)-bonded structure, while all other dications favor the pi-bonded Se2I42+ structure. Theoretical bond order calculations for S2I42+ confirm the previously presented experimentally based bond orders for S-S (2.1-2.3) and I-I (1.3-1.5) bonds. The S-S bond is determined to have the highest reported S-S bond order in an isolated compound and has a bond order that is either similar to or slightly less than the Si-Si bond order in the proposed triply bonded [(Me3Si)2CH]2(iPr)SiSi triple bond SiSi(iPr)[CH(SiMe3)2]2 depending on the definition of bond orders used.
RESUMO
The vibrational spectra of S2I4(MF6)2(s) (M = As, Sb), a normal coordinate analysis of S2I4(2+), and a redetermination of the X-ray structure of S2I4(AsF6)2 at low temperature show that the S-S bond in S2I4(2+) has an experimentally based bond order of 2.2-2.4, not distinguishably different from bond orders, based on calculations, of the Si-Si bonds in the proposed triply bonded disilyne of the isolated [(Me3Si)2 CH]2 (iPr)SiSiSiSi(iPr)[CH(SiMe3)2]2 and the hypothetical trans-RSiSiR (R = H, Me, Ph). Therefore, both S2I4(2+) and [(Me3Si)2 CH]2 (iPr)SiSiSiSi(iPr)[CH(SiMe3)2]2 have the highest bond orders between heavier main-group elements in an isolated compound, given a lack of the general acceptance of a bond order > 2 for the Ga-Ga bond in Na2[{Ga(C6H3Trip2-2,6)}2] (Trip = C6H2Pr(i)3-2,4,6) and the fact that the reported bond orders for the heavier group 14 alkyne analogues of formula REER [E = Ge, Sn, or Pb; R = bulky organic group] are ca. 2 or less. The redetermination of the X-ray structure gave a higher accuracy for the short S-S [1.842(4) A, Pauling bond order (BO) = 2.4] and I-I [2.6026(9) A, BO = 1.3] bonds and allowed the correct modeling of the AsF6- anions, the determination of the cation-anion contacts, and thus an empirical estimate of the positive charge on the sulfur and iodine atoms. FT-Raman and IR spectra of both salts, obtained for the first time, were assigned with the aid of density functional theory calculations and gave a stretching frequency of 734 cm(-1) for the S-S bond and 227 cm(-1) for the I-I bond, implying bond orders of 2.2 and 1.3, respectively. A normal-coordinate analysis showed that no mixing occurs and yielded force constants for the S-S (5.08 mdyn/A) and I-I bonds (1.95 mdyn/A), with corresponding bond orders of 2.2 for the S-S bond and 1.3 for the I-I bond, showing that S2I4(2+) maximizes pi bond formation. The stability of S2I4(2+) in the gas phase, in SO2 and HSO3F solutions, and in the solid state as its AsF6- salts was established by calculations using different methods and basis sets, estimating lattice enthalpies, and calculating solvation energies. Dissociation reactions of S2I4(2+) into various small monocations in the gas phase are favored [e.g., S2I4(2+)(g) --> 2SI2(+)(g), deltaH = -200 kJ/mol], as are reactions with I2 [S2I4(2+)(g) + I2(g) --> 2SI3(+)(g), deltaH = -285 kJ/mol). However, the corresponding reactions in the solid state are endothermic [S2I4(AsF6)2(s) --> 2SI2(AsF6)(s), deltaH = +224 kJ/mol; S2I4(AsF6)2 + I2(s) -->2SI3(AsF6)(s), deltaH = +287 kJ/mol). Thus, S2I4(2+) and its multiple bonds are lattice stabilized in the solid state. Computational and FT-Raman results for solution behavior are less clear cut; however, S2I4(2+) was observed by FT-Raman spectroscopy in a solution of HSO3F/AsF5, consistent with the calculated small, positive free energies of dissociation in HSO3F.
