DEFORMATION ELECTRON-DENSITY DISTRIBUTIONS OF TETRAAZATHIAPENTALENES WITH HYPERVALENT S-N BONDS

3,4-Dimethyl-2,5-diphenyl-3,4-dihydro-3a-thia-1,3,4,6-tetraazapentalene (1) and 2,3,4,5-tetrahydro-1,6-diphenyl-3,4-propano-6a-thia-1,3,4,6-tetraazapentalene-2,5-dione (2) are typical hypervalent sulfur compounds with S-N bonds of 1.90-1.96 Angstrom, which are longer than the normal single S-N bond (1.74 Angstrom) by about 10%. The electron-density distributions of these compounds were investigated in order to shed light on the character of hypervalent S...N bonds. Intensity data of X-ray diffraction were measured at 143 K and the structure refinements were performed using multipole expansion atomic scattering factors up to the hexadecapole expansion for the S atom. Crystal data are: 1, orthorhombic, Pbcn, a = 14.433(3), b = 9.220(2), c = 11.236(2) Angstrom, Z = 4, R = 0.037 for 6398 reflections; 2, orthorhombic, Iba 2, a = 20.313(3), b = 21.365(2), c = 7.472(2) Angstrom, Z = 8, R = 0.036 for 6617 reflections. In the model deformation maps of 1 and 2 electron densities along the S-N bonds are observed near the nitrogen atoms, not in the center of the bonds. The net atomic charges derived from the multipole refinement suggest a polarized character of the S-N bond. Lone-pair electron densities of hypervalent sulfur atoms, observed on the up and down sides of the pentalene plane, suggest a coupling of lone pair with p pi and non-bonding d or p electrons.