Sulfur tetrafluoride geometry

Sulfur tetrafluoride has two possible molecular geometries. The stable isomer is the seesaw form. 

Sulfur tetrafluoride appears as two broad singlets at room temperature, as one broad singlet at 85 °C, and (when dry) as two sharp triplets at -30 °C. SF6, with its symmetrical octahedral geometry, appears as a sharp singlet at all temperatures. The activation energy for pseudorotation of SF4, which interconverts its axial and equatorial fluorines, is 12kcalmor1.3... 

Molecular geometries other than trigonal bipyramidal are possible when one or more of the five electron pairs are lone pairs. Consider the sulfur tetrafluoride molecule, SF4. The Lewis electron-dot formula is... 

Thus you expect lone pairs to occupy equatorial positions in the trigonal bipyramidal arrangement. For sulfur tetrafluoride, this gives a seesaw (or distorted tetrahedral) geometry (Figure 10.10). 

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