The Structures of [X2YNYX2]+Cations (X F, Cl; Y S, Se). A Comparison of X-ray and ab Initio Studies[1]

The structures of cationic species of the series [X2YNYX2]+ (X F, Cl; Y S, Se) have been computed ab initio using all electron treatments for first-row elements and sulfur and quasi-relativistic pseudopotentials for Se and Cl. Splitvalence basis sets with polarization and diffuse functions were employed. The MP2 results for the (non-isostructural!) cations [Cl2SeNSeCl2]+ (1: Cs) and [F2SNSF2]+ (2: C2v) are in excellent agreement with the experimental (X-ray) observations. Both structures represent local minima. A deeper minimum for either of the cations is represented by another C2v isomer which for crystal lattice energy reasons is stable in the isolated state only. The geometries of the hitherto unknown species [Cl2SNSCl2]+ (3) and [F2SeNSeF2]+ (4) have been assessed by ab initio HF calculations. In analogy to 2, cations 3 and 4 are predicted to prefer C2v symmetry. Therefore, 1 exhibits unusual structural features. According to strictly localized natural bond orbital analysis (NBO), the central nitrogen atoms in 1 and 2 possess two lone pairs of electrons (LP: one sp hybrid and one p orbital). The relatively short SeN and SN bond distances in 1 (1.741–1.760 A) and 2 (1.551 A) can best be attributed to LP(N)s*(YX) negative hyperconjugation (1: Y Se, X Cl; 2: Y S, X F).