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 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.

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