Density functional study of tetra-atomic clusters and complexes of the group 16 elements: Trends in structure and bonding

Se(4) and Te(4) clusters, the intermolecular weakly bound charge-transfer complexes (Se(2))(O(2)) and (Te(2))(O(2)), and the (X(2))(Y(2)) and (XY)(Y(2)) mixed compounds (X, Y = S, Se, Te) were studied using density functional theory with all-electron and effective core potential basis sets. All systems were analyzed in terms of interactions of the diatomic molecules. The energetically favorable "face to face" binding of the diatomic moieties leads to delocalization of the electron density in the high-lying occupied pi*-antibonding orbitals of the tetra-atomic products. The cis structures are minima for all systems considered asa result of this delocalization. For the clusters and mixed compounds, the cia tetra-atomic structures formed by the diatomic molecules with equal or similar electronegativities correspond to global minima. The diatomic fragments in these compounds are nonparallel and have shorter bond lengths than their free diatomic values. For the clusters, the other low-lying isomers ordered according to their relative stabilities are trans, branched ring, and puckered ring. The increase in the delocalization of bond lengths leads to a decrease in relative stabilities. The rectangular configuration was predicted to be a transition state in the degenerate interconversion of two cis forms of Sed, with a chemically insignificant barrier. For Te(4), this barrier disappears and the geometry of the cis global mimima is extremely close to the rectangular structure. The stability of the (XY)(YX) compounds with heteronuclear diatomic fragments relative to the (X(2))(Y(2)) isomers with the homonuclear diatomic species increases with a decrease in the difference between the electronegativities of the X and Y atoms. The cis structures of (Se(2))(O(2)) and (Te(2))(O(2)) with the larger difference in electronegativities between the diatomic moieties correspond to local minima.