Bonding in YXXY dihalides and dihydrides of dioxygen and disulfur

Abstract

Ab initio calculations based on spin-coupled theory are used to compare and contrast the bonding in FOOF, HOOH, FSSF, CISSCI and HSSH, with emphasis on rationalising the striking variations in bond length. The resulting modern VB descriptions of the X—Y σ-like bonds closely parallel those for the analogous XY₂ species, except for the expected changes to the orbital overlaps induced by the different bond lengths. In examining the variations in geometry, it seems that the most relevant aspect of the bonding is that provided by the various p_π-like orbitals. In FOOF, FSSF and, to a lesser extent, CISSCI, incipient hypercoordinate character is observed (at oxygen or sulfur), with two partial π-like interactions in approximately perpendicular planes, as well as some antibonding character in the X—Y bonds.