PREPARATION, CHARACTERIZATION, AND X-RAY CRYSTAL-STRUCTURES OF S2I4(MF6)2 (M = AS, SB) AND THE ELECTRONIC-STRUCTURE OF THE HIGHLY PI-BONDED ECLIPSED DISULFUR TETRAIODINE (2+) CATION - THERMODYNAMIC CONSIDERATIONS AND THE NONEXISTENCE OF SI3ASF6(S)

Crystalline S2I4(AsF6)2 was prepared quantitatively by the reaction of stoichiometric amounts of S8, I2, and AsF5, in liquid SO2, and in reactions designed to give SI3AsF6(s). It was also a product of reactions designed to give various other related sulfur-iodine cations. Crystalline S2I4(SbF6)2 was prepared essentially quantitatively by the reaction of stoichiometric amounts of S8, I2, and SbF5, designed to give S2I4(SbF6)2 and 5SbF3.3SbF5. Crystal data for S2I4(AsF6)2: monoclinic, space group C2/c (No. 15), a = 9.650 (2) angstrom, b = 12.874 (2) angstrom, c = 13.644 (1) angstrom, beta = 93.79 (2)-degrees, Z = 4, R = 0.049 (R(w) = 0.054) for 948 significant reflections. Crystal data for S2I4(SbF6)2: orthorhombic, space group Cmcm (No. 63), a = 9.850 (4) angstrom, b = 13.067 (9) angstrom, c = 13.782 (9) angstrom, Z = 4, R = 0.051 (R(w) = 0.058) for 568 significant reflections. The S2I4(MF6)2 salt consist of S2I4(2+) and MF6-[M = As, Sb] with weak cation-anion interactions. The S2I4(2+) cations have distorted right-triangular-prismatic structures, with symmetry C2 (AsF6-) and C2-upsilon (SbF6-). Each cation consists of two quadrilateral S2I2 units joined at the common S-S bond, with interplanar angles of 89.8 (1)-degrees (AsF6-) and 91.3 (1)-degrees (SbF6-). The S-S distances of 1.818 (10) angstrom (SbF6-) and 1.843 (6) angstrom (AsF6-) are the shortest reported for an isolated compound and correspond to bond orders of 2.7 and 2.4, respectively. The I-I bonds are 2.571 (2) angstrom (SbF6-) and 2.5987 (17) angstrom (AsF6-), corresponding to bond orders 1.4 and 1.3. The S-I bond distances are 2.993 (4) angstrom (SbF6-) and 2.860 (4) and 3.178 (4) angstrom (AsF6-). A simple model for the bonding in S2I4(2+) is proposed in which an S2 unit is bonded to two I2+ units by two mutually perpendicular four-center-two-electron pi*-pi* bonds, with some charge redistribution resulting in a +0.33 charge on each of the six atoms. This model is supported by quantum-mechanical STO-3G ab initio calculations on rectangular Cl4(2+), as a two dimensional model of the bonding in S2I4(2+), and calculations on O2Cl4(2+) of suitable geometry. The enthalpy of the formation of S2I4(AsF6)2(s) from corresponding amounts of S8, I2, and AsF5 is estimated by a simple model to be much more favorable than that of 2SI3AsF6(s). In addition, the enthalpy of disproportionation of the hypothetical 2SI3AsF6(s) to S2I4(AsF6)2(s) and I2(s) is estimated to be substantially exothermic and the enthalpy of isomerization of the observed pi-bonded S2I4(2+) to the classical sigma-bonded I2S+S+I2 is also estimated to be endothermic.