A comparison between polar covalent bonding and hypervalent recoupled pair bonding in diatomic chalcogen halide species {O,S,Se} x {F,Cl,Br}

The (2)Pi ground states and low-lying (4)Sigma excited states of the nine diatomic species formed by combining O, S, or Se with F, Cl, or Br were characterized with multireference configuration interaction calculations in order to compare and contrast the behaviour of the bonding in these two states of the chalcogen halides. For each of these species, the (2)Pi ground state is polar covalently bound via simple singlet coupling of unpaired electrons on each atom. But for each compound there is also a bound (4)Sigma(-) state where bond formation requires recoupling the valence p(2) pair of electrons on the chalcogen atom. This mode of bonding makes more electrons available for additional bond formation and is the basis of hypercoordination. The behaviour of S and Se differs significantly from that of O. Although the hypervalent (4)Sigma(-) states of the diatomic chalcogen halides are bound for all three elements, the O species are only weakly bound and exhibit minimal recoupling compared with the corresponding S and Se compounds.