PREPARATIONS AND CRYSTAL-STRUCTURES OF SE2I4(ASF6)2.SO2 AND SE2I4(SB2F11)2 AND ELECTRONIC-STRUCTURE OF THE ECLIPSED DISELENIUM TETRAIODIDE DICATION, CONTAINING 2 SEI2.+ RADICAL CATIONS JOINED VIA A PI-STAR PI-STAR BOND AND A DELOCALIZED 4P-PI5P-PI BOND - IMPLICATIONS FOR BONDING IN S2O42-, SE-8(2+), AND S-8(2+)

Se2I4(AsF6)2·SO2 and Se2I4(Sb2F11)2 were prepared quantitatively by reacting stoichiometric amounts of Se, I2, and AsF5 and by reacting Se and I2Sb2F11 in liquid SO2, respectively. The 77Se NMR spectra (-70 °C) of the salts in SO2 solution indicated the presence of Se4J+, SeI3+, and an unidentified peak at 1345 ppm (referenced to neat (CH3)2Se at room temperature). Crystal data for Se2I4(AsF6)2·SO2: monoclinic, space group P21/c, a = 13.454 (2) Å, b = 9.930 (1) Å, c = 14.576 (1) Å, β = 93.20 (1)°, Z = 4. R = 0.043 for 1045 significant reflections. Crystal data for Se2I4(Sb2F11)2: triclinic, space group [formula omitted], a = 17.915 (1) Å, 6 = 9.276 (1) Å, c = 8.001 (1) Å, α = 96.04 (l)°, β = 95.22 (1)°, γ = 91.83 (l)°, Z = 2. R = 0.046 for 2647 significant reflections. The crystal packing of Se2I4(AsF6)2-2SO2 and Se2I4(Sb2F11)2 consists of columns of Se2I42+, AsF6“, and SO2 and columns of Se2I42+ and Sb2F11-, respectively. The Se2I42+ cations in both salts are similar and adopt the eclipsed configuration with long Se-Se bonds (average 2.841 (6) Å). There are significant intraionic interactions between the syn iodine atoms (average 3.673 (4) Å) that give the cation a distorted trigonal-prism-like shape. The Se-I bonds (average 2.451 (5) Å) are shorter than those in Sel3+ (average 2.510 (2) Å), implying 4pπ-5pπ bonding, with a formal bond order of 1.25 per Se-I bond. We conclude that Se2I42+ consists of two Sel2·+ radical cations joined by a 6-center, 2-electron π*-π* bond. This view is supported by ab initio (STO-3G) calculations on S2Cl42+ as a model for Se2I42+. The geometries of the related Se82+, Sg2+, and S2O42- ions imply that they also contain similar π*-π* bonds. The behavior of Se2I4(AsF6)2 and Se2I4(Sb2Fu)2 in SO2 solution is discussed. © 1990, American Chemical Society. All rights reserved.