Computational study of the electronic and geometric structures of the dihalogenodimethylselenium compounds, Me2SeX2 (X = F, Cl, Br, I or At)†

Abstract

The dihalogenodimethylselenium compounds Me₂SeX₂ (X = F, Cl, Br, I or At) have been studied computationally using quasi-relativistic density functional theory. For Me₂SeX₂ (X = F, Cl or Br), the most stable structure is calculated to be the hyperco-ordinate C_2v symmetric ψ-trigonal-bipyramidal geometry. By contrast, the charge-transfer C_s-symmetric ‘spoke’ structures, which obey the octet rule, are found to be the most stable form of Me₂SeI₂ and Me₂SeAt₂. This change in structural type with increasing halogen atomic number is in agreement with previous X-ray crystallographic studies. The factors that determine the most stable structure are found to be intramolecular in origin. The preference for the ψ-trigonal-bipyramidal geometry of Me₂SeX₂ (X = F, Cl or Br) is traced to the much greater bond-strength difference between Se–X and X–X for the lighter halogens. Analysis of the intramolecular charge distributions and molecular orbital compositions indicated that the Se–X bond polarity decreases significantly with increasing halogen atomic number. This is in agreement with previous theoretical studies, which indicate that hyperco-ordinate systems are favoured by highly polar bonds to the central atom. No evidence was found for significant involvement of the selenium d orbitals in the Se–X or Se–C bonding in hyperco-ordinate Me₂SeX₂.