Coordination of Li+, Ca+, V+, and Cu+ to the Molecules S8 and S4 –A Computational Study

The complex formation between the Li+ cation and the sulfur homocycle S8 has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Starting with various isomers of S8, the formation of LiS8 heterocycles and clusters is preferred over complexes with a monodentate ligand. The binding energies of the cation in the 23 complexes investigated range from –95 to –217 kJ·mol–1. The global minimum structure of [LiS8]+ is of C4v symmetry with the S8 homocycle in the well-known crown conformation and four Li–S bonds of length 254.2 pm (binding energy: –156.5 kJ·mol–1). The S–S bonds of the various ligands are slightly weakened by the complex formation and a more or less strong bond length alternation is induced. Relatively unstable isomers of S8 (chair, tub, exo–endo ring, branched rings, triplet chain) are partly stabilized and partly destabilized by complex formation with Li+. The interaction between the cation and the S8 ligands is mainly due to ion–dipole attraction with little to moderate charge transfer (0.04–0.27 electrostatic units). In the four most stable isomers of [LiS8]+, the number of sulfur–sulfur bonds is at a maximum and the coordination number of Li+ is either 4 or 3. Complexes of the type [Li(S4)2]+ are much less stable than isomers with an eight-atomic ligand. The Li–S bond lengths in all of these complex cations (230–273 pm) depend on the coordination number of Li and on the atomic charge of the donating sulfur atom(s). In contrast to [LiS8]+, the complexes of composition [MS8]+ with M = Ca, V, and Cu are more stable as [M(S4)2]+ than with an eight-atomic crown-shaped ligand. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)