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 pi-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 pi-bonds to the sigma*(SF) antibonding orbitals. In the case of SF2, a sulfur lone pair can transfer charge into the pi* MOs of the alkene, a back-transfer which is more difficult for SF4.