Thallium halides - New aspects of the stereochemical activity of electron lone pairs of heavier main-group elements

Density functional theory studies in the linear muffin tin approximation on thallium halides (TIX; X = F, Cl, Br, I) show that structural distortions due to a stereochemically active lone pair of electrons depend crucially on cation-anion interactions. Such distortions do not originate from a 6s-6p hybridisation on the heavy metal as the TI 6s and 6p states are energetically too far apart to mix directly. In the case of TIF, the TI 6s and the F 2p states are located in the same energy region, which leads to an efficient interaction that produces strongly antibonding TI-F states directly below the Fermi level that would destabilize the solid. Minimisation of these unfavourable covalent antibonding interactions is the driving force for structural distortion. In the case of the heavier thallium halides, the growing energetic mismatch between Tl 6s states and X np states leads to weaker covalent interactions. Thus, the compounds become more ionic and high symmetry structures are adopted as covalent interactions play only a minor role. The low symmetry structure of TII observed at low temperatures is stabilized by attractive metal-metal bonding. (c) Wiley-VCH Verlag GmbH & Co. KGaA.