The highest bond order between heavier main-group elements in an isolated compound?: Energetics and vibrational spectroscopy of S2I4(MF6)2 (M = As, Sb)

The vibrational spectra of S2I4(MF6)(2)(s) (M = As, Sb), a normal coordinate analysis of S2I42+, and a redetermination of the X-ray structure of S2I4(AsF6)(2) at low temperature show that the S-S bond in S2I42+ 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)(Pr-i)SiSiSiSi(Pr-i)[CH(SiMe3)(2)](2) and the hypothetical trans-RSiSiR (R = H, Me, Ph). Therefore, both S2I42+ and [(Me3Si)(2)CH](2)(Pr-i)SiSiSiSi(Pr-i)[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 Na-2[{Ga(C(6)H(3)TriP(2)-2,6)}(2)] (Trip = C6H2Pr3i-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) angstrom, Pauling bond order (130) = 2.4] and I-I [2.6026(g) angstrom, 13] = 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/angstrom) and I-I bonds (1.95 mdyn/angstrom), with corresponding bond orders of 2.2 for the S-S bond and 1.3 for the I-I bond, showing that S2I42+ maximizes T bond formation. The stability of S2I42+ 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 S2I42+ into various small monocations in the gas phase are favored [e.g., S2I42+(g) -> 2SI(2)(+)(g), Delta H = -200 kJ/mol], as are reactions with I-2 [S2I42+(g) + I-2(g) -> 2SI(3)(+)(g), Delta H = -285 kJ/mol). However, the corresponding reactions in the solid state are endothermic [S2I4(AsF6)(2)(s) -> 2SI(2)(AsF6)(s), Delta H = +224 kJ/mol; S2I4(AsF6)(2) + I-2(s) -> 2SI(3)(AsF6)(s), Delta H = +287 kJ/mol). Thus, S2I42+ and its multiple bonds are lattice stabilized in the solid state. Computational and T-Raman results for solution behavior are less clear cut; however, S2I42+ was observed by FT-Raman spectroscopy in a solution of HSO3F/AsF5, consistent with the calculated small, positive free energies of dissociation in HSO3F.