STRUCTURE AND BONDING IN SQUARE-PLANAR CHALCOGEN RINGS

The molecular structures of square-planar X4(2+), X4(+), and X4 (X = S, Se, Te) have been calculated using the effective core potential model. For X4(2+) the agreement between experimental and calculated values is excellent provided that d orbitals are included in the basis set. For the hypothetical molecules X4(+) and X4 the bond lengths are found to increase dramatically as one and, subsequently, two electrons are added to the systems. Extensive population analysis shows that this increase is almost exclusively due to loss of bonding in the pi-system, whereas the bonding in the sigma-system remains relatively unaltered. These results make it possible to predict covalent single bond radii for S, Se, and Te for which the influence of pi-repulsion is removed. From the calculated variation of bond lengths with atomic charge, bond lengths are predicted for a series of planar disulphide rings.