Hypervalency in sulfur? ab initio and DFT studies of the structures of thiosulfate and related sulfur oxyanions

The structures of thiosulfate (S2O3 2−) and four other isovalent sulfur oxyanions, S m O n 2− have been investigated at the MP2, MP4, and B3LYP levels of theory, with 6−31+G* as the basis set. By comparing the bond lengths in the dianions with those in the doubly protonated molecules, H2S m O n , the sulfur–oxygen and sulfur–sulfur bond orders have been estimated, based on a valence bond theory model. The conventional Lewis structures, which represent the central S atom as forming six covalent bonds with the four ligands, consistently give accurate descriptions of the structures for all the dianions. Natural bond order analyses, however, show that the 3d orbitals do not participate in valence bonding to any significant degree. Chemically, the peripheral sulfur atoms in all the dianions are predicted to be stronger nucleophiles as well as stronger bases than the oxygen atoms in the gas phase.