THE CLSENSECL- AN UNUSUAL STRUCTURALLY VERY FLEXIBLE ION ADOPTING DIFFERENT SOLID-STATE STRUCTURES AS DEDUCED BY X-RAY AND AB-INITIO METHODS( CATION )

The new salt [ClSeNSeCl]+[SbCl6](-)was prepared in 89% yield from the reaction of [SeCl3](+)[SbCl6](-)and N(SiMe(3))(3)). This method provid es the first synthesis that is suitable for the preparation of a [ClSe NSeCl](+) salt on a preparative scale. The new salt was characterized by elemental analysis and vibrational (IR, Raman), Se-77 NMR, and mass spectroscopy. The low-temperature structure was determined by a singl e-crystal X-ray diffraction analysis. The compound crystallizes in the orthorhombic space group P2(1)nb with Z = 8 and unit cell dimensions a = 7.0068(17), b = 14.6678(23), and c = 37.365(13) Angstrom. The stru cture is made up of octahedral SbCl6- anions and s-shaped (s-isomer) [ ClSeNSeCl](+) cations with some cation-anion interactions. In the [ClS eNSeCl](+) cation the two Se-N bond lengths (1.644(21), 1.753(22) Angs trom) are just slightly different (double esd's) in agreement with qua ntum mechanical computations predicting that the asymmetrical geometry provides a better net positive overlap for the LP(N) -->sigma(Se-Cl) negative hyperconjugation (LP,lone pair). The structures of the catio nic species df the series [ClXNXCl](+) (X = S,Se) were determined ab i nitio using all-electron computations for first-row elements and sulfu r and quasi relativistic pseudopotentials for Se and Cl. Split valence basis sets with polarization and diffuse functions (6-31+G and dz+p, respectively) were employed. The MP2 results for the u-isomer (C-2 up silon) of the cations [ClXNXCl](+) (X = S,Se) are in excellent agreeme nt with the experimental (X-ray) XNX angles: [ClSeNSeCl](+)[GaCl4](-), 146.6(3)degrees (X-ray), 142.5 degrees (MP2); [ClSNSCl](+)[AlCl4](-), 151.0(2)degrees (X-ray), 152.2 degrees (MP2). The novel structural s- isomer (C-s) of the [ClSeNSeCl](+) cation in the new salt [ClSeNSeCl]( +) [SbCl6](-) and the hitherto unknown w-isomer (C-2 upsilon) were ful ly optimized with the MP2/6-31+G basis sets. It was shown that both e xperimentally observed isomers (u- and s-isomers) of [ClSeNSeCl](+) ar e essentially identical in their total energy (Delta E = 1 kcal/mol in favor for the u-isomer) whereas the only other local minimum (w-isome r) was found to be 9.2 kcal/mol less stable than the u-isomer. Frequen cy calculations were performed at MP2/6-31+G on the optimized structu res for both the u-and the s-isomers of both salts [ClXNXCl](+) (X = S ,Se). In general, agreement between the calculated and observed geomet ries and frequencies is very good at the correlated MP2 level.