ENERGETICS OF FORMATION AND X-RAY CRYSTAL-STRUCTURES OF SNSNS(MF(6))(2) (M=AS, SB), CONTAINING THE LATTICE-STABILIZED, AROMATIC, 6-PI 1,3,4,2,5-TRITHIADIAZOLIUM(2+) CATION FORMED BY THE CRYSTAL-LATTICE-ENFORCED SYMMETRY-ALLOWED CYCLOADDITION OF SN(+) AND SNS(+)

SNSNS(AsF6)(2), formed by the lattice-enforced symmetry-allowed cycloaddition of SN+ and SNS+, was obtained in quantitative yield by the evaporation of a solution of SNAsF6 and SNSAsF6 in SO2 and characterized by vibrational spectroscopy and X-ray crystallography. Crystal data: space group C2 (No. 5), a = 12.558(2) Angstrom, b = 8.454(1) Angstrom, c = 10.583(2) Angstrom, beta = 92.51(2)degrees, Z = 4, R = 3.98% (R(W) = 4.71%) for 770 observed reflections and 163 independent parameters. Although it has long been predicted to be a stable, 6 pi, electron-rich aromatic, the SNSNS2+ cation dissociates reversibly in SO2 solution (N-14 NMR), but is stable in the solid state in SNSNS(MF(6))(2) (M = As, Sb). This is consistent with the results of fully optimized 6-31C* MO calculations, from which Delta H[SN+(g) + SNS+(g) --> SNSNS2+(g)] = +400 kJ mol(-1). However, in the solid state, the high lattice energy of SNSNS(AsF6)(2) (1493 2kJ mol(-1)) compared to the combined lattice energies of SNAsF6 + SNSAsF6 (1065 kJ mol(-1)) stabilizes the 1:2 salt over the two 1:1 salts. In SO2 solution simple thermochemical estimates suggest that SNS+(SO2) and SN+(SO2) are marginally more stable than SNSNS2+(SO2). This is consistent with the observed stability in SO2 of the related selenium containing cations SexS3-xN22+ (x = 1-3), which results from the well-known preference of Se for sigma-bonded structures. The similarity of SexS3-xN22+ cations is supported by a normal coordinate analysis of their vibrational spectra (for x = 0, 2, 3) which shows that a consistent set of force constants characterizes their force fields. The stability of salts of SNSNS2+ in the solid state depends critically upon the lattice energy, which is reduced in the presence of large counteranions. Consistently, while we were able to prepare SNSNS(SbF6)(2) (Raman, X-ray crystallography), an attempt to obtain SNSNS(Sb2F11)(2) gave only a mixture of SNS(Sb2F11) and SN(Sb2F11) (Raman). The structure of SNSNS(SbF6)(2) was determined by single crystal X-ray diffraction: monoclinic space group P2(1)/n, a = 12.928(3) Angstrom, b = 31.619(7) Angstrom, c = 8.778(5) Angstrom, beta = 90.26(3)degrees, V = 3588(2) Angstrom(3), Z = 12, which is a supercell of an orthorhombic Pna2(1) system (a = 10.544(2) Angstrom, b = 12.952(3) Angstrom, c = 8.801(4) Angstrom, V = 1201.9(7) Angstrom(3), Z = 4). This supercell probably extends to a yet larger supercell which has not been examined.