The molecular structure of gaseous bis(trifluoromethyl)selenium difluoride as determined by electron diffraction

Abstract

The structure of gaseous bis(trifluoromethyl)selenium difluoride has been determined by electron diffraction. Hence the predominant species is shown to be the monomer (CF₃)₂SeF₂ with the CF₃ ligands occupying the equatorial sites of a framework derived from a trigonal bipyramid. Salient structural parameters (r_a) are: r(Se–F) 182.7(0.5), r(Se–C) 202.2(0.8), and r(C–F) 131.4(0.3) pm; F–Se–F 157.8(3.5), C-Se–C 118.7(1.7), and F–C–Se 108.9(0.4)°. The molecule has C₂ symmetry overall, with the CF₃ groups twisted 19.8(1.1)° away from the positions in which one C–F bond of each group is anti with respect to an Se–C bond (corresponding to a conformation having C_2V symmetry overall). Comparisons with the structures of related molecules show that the switch from SeF₄ to (CF₃)₂SeF₂, unlike that from SF₄ to (CF₃)₂SF₂, results in an increase in the equatorial bond angle, in keeping with the expectations of the valence shell electron pair repulsion model.