S···π Chalcogen Bonds between SF₂ or SF₄ and C–C Multiple Bonds

SF₂ and SF₄ were each paired with a series of unsaturated hydrocarbons including ethene, ethyne, 1,3-butadiene, and benzene, in each case forming a chalcogen bond between the S atom and the carbon π-system. MP2 ab initio calculations reveal that the S atom is situated directly above one specific CC bond, even when more than one are present. The binding energies range between 3.3 and 6.6 kcal/mol. SF₂ engages in a stronger, and shorter, noncovalent bond than does SF₄ for all systems with the exception of benzene, to which SF₄ is more tightly bound. cis-Butadiene complexes contain the shortest chalcogen bond, even if not necessarily the strongest. The internal S–F covalent bonds elongate upon formation of each chalcogen bond. The molecules are held together largely by charge transfer forces, particularly from the CC π-bonds to the σ*­(SF) antibonding orbitals. In the case of SF₂, a sulfur lone pair can transfer charge into the π* MOs of the alkene, a back-transfer which is more difficult for SF₄.