WHY Tecl4 is more stable than Scl4

Structural and conformational properties of the tetrahalides MX₄with M = S,Se,Teand X =F,Cl,Br, I were studied with quantum chemical calculations (MP2/aug-cc-pVTZ and relativistic core potentials for heavy atoms). The pseudotrigonal bipyramidal geometries (C_2vsymmetry) of the three tetrafluorides and ofTeCl₄, whose structures have been determined by gaselectron diffraction, are very well reproduced with this computational method. No additional stable conformer withC_3v,C_S,C_4vorT_dsymmetry is predicted for these fourchalcogentetrahalides. For all other MX₄compounds, except SeI₄, the existence of two conformers withC_2vandT_dsymmetry is predicted. TheT_dstructure is favoured in the case of SI₄andSeBr₄and SeI₄is predicted to exist only asT_dconformer.