Electrophilic Attack on Sulfur–Sulfur Bonds: Coordination of Lithium Cations to Sulfur‐Rich Molecules Studied by Ab Initio MO Methods

Complex formation between gaseous Li + ions and sulfur‐containing neutral ligands, such as H 2 S, Me 2 S n ( n = 1–5; Me = CH 3 ) and various isomers of hexasulfur (S 6 ), has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Generally, the formation of LiS n 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 2 SLi] + to −189 kJ mol −1 for the most stable isomer of [Me 2 S 5 Li] + which contains three‐coordinate Li + . Of the various S 6 ligands (chair, boat, prism, branched ring, and triplet chain structures), two isomeric complexes containing the S 5 S ligand have the highest binding energies (−163±1 kJ mol −1 ). However, the global minimum structure of [LiS 6 ] + is of C 3 v symmetry with the six‐membered S 6 homocycle in the well‐known chair conformation and three LiS bonds with a length of 256 pm (binding energy: −134 kJ mol −1 ). Relatively unstable isomers of S 6 are stabilized by complex formation with Li + . The interaction between the cation and the S 6 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 S 2 , S 3 , or S 4 units, as in [Li(S 3 ) 2 ] + and [Li(S 2 )(S 4 )] + . In the two most stable isomers of the [LiS 6 ] + complexes, the number of SS 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 LiS bonds of lengths below 280 pm are characterized by a maximum electron‐density path and closed‐shell interaction.