BONDING IN YXXY DIHALIDES AND DIHYDRIDES OF DIOXYGEN AND DISULFUR

Ab initio calculations based on spin-coupled theory are used to compar e 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 sigma-like bonds closely p arallel those for the analogous XY(2) 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 rele vant aspect of, the bonding is that provided by the various p(pi)-like orbitals. In FOOF, FSSF and, to a lesser extent, CISSCI, incipient hy percoordinate character is observed (at oxygen or sulfur), with two pa rtial pi-like interactions in approximately perpendicular planes, as w ell as some antibonding character in the X-Y bonds.