Electrophilic attack on sulfur-sulfur bonds: Coordination of lithium cations to sulfur-rich molecules studied by ab initio MO methods

Abstract

Complex formation between gaseous Li+ ions and sulfur-containing neutral ligands, such as H2S, Me2Sn (n = 1-5; Me = CH3) and various isomers of hexasulfur (S6), has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Generally, the formation of LiSn heterocycles and clusters is preferred in these reactions. The binding energies of the cation in the 29 complexes investigated range from -88 kJ mol-1 for [H 2SLi]+ to -189 kJ mol-1 for the most stable isomer of [Me2S5Li]+ which contains three-coordinate Li+. Of the various S6 ligands (chair, boat, prism, branched ring, and triplet chain structures), two isomeric complexes containing the S5=S ligand have the highest binding energies (-163±1 kJ mol-1). However, the global minimum structure of [LiS 6]+ is of C3v symmetry with the six-membered S6 homocycle in the well-known chair conformation and three Li-S bonds with a length of 256 pm (binding energy: -134 kJ mol-1). Relatively unstable isomers of S6 are stabilized by complex formation with Li+. The interaction between the cation and the S6 ligands is mainly attributed to ion-dipole attraction with a little charge transfer, except in cations containing the six sulfur atoms in the form of separated neutral S2, S3, or S4 units, as in [Li(S3)2]+ and [Li(S2)(S 4)]+. In the two most stable isomers of the [LiS 6]+ complexes, the number of S-S bonds is at maximum and the coordination number of Li+ is either 3 or 4. A topological analysis of all investigated complexes revealed that the Li-S bonds of lengths below 280 pm are characterized by a maximum electron-density path and closed-shell interaction. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.