Coordination of Li +, Ca +, V +, and Cu + to the molecules S 8 and S 4 -A computational study

Abstract

The complex formation between the Li + cation and the sulfur homocycle S 8 has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Starting with various isomers of S 8, the formation of LiS 8 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 [LiS 8] + is of C 4v symmetry with the S 8 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 SS 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 S 8 (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 S 8 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 [LiS 8] +, 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(S 4) 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 [LiS 8] +, the complexes of composition [MS 8] + with M = Ca, V, and Cu are more stable as [M(S 4) 2] + than with an eight-atomic crown-shaped ligand. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.