Sulfur hexafluoride, electronic structure

The largest class of molecules to violate the octet rule consists of species in which the central atom is surrounded by more than four pairs of valence electrons. Typical molecules of this type are phosphorus pentachloride, PC15, and sulfur hexafluoride, SF6. The Lewis structures of these molecules are... 

Sulfur hexafluoride sublimes at -64 °C to produce a dense gas (6.14 g L-1). Under a pressure of 2 atm, the melting point is -51 °C. The molecule has the expected octahedral structure and a dipole moment of zero. The compound is so inert that it is used as a gaseous insulator, and rats allowed to breathe a mixture of SF6 and oxygen show no ill effects after several hours of exposure. This inertness is a result of the molecule having no vacant bonding site or unshared electron pairs on sulfur to initiate a reaction and the fact that six fluorine atoms shield the sulfur atom from attack. Consequently, there is no low-energy pathway for reactions to occur, and the compound is inert even though many reactions are thermodynamically favored. 

Some atoms appear to exceed the octet rule. This behavior is observed only for those elements in Period 3 of the periodic table and beyond. To see how this arises, we will consider the Lewis structure for sulfur hexafluoride (SF6). The sum of the valence electrons for SF6 is... 

The molecule sulfur hexafluoride (SFA has recently challenged both molecular spectroscopy with its unexpected rotational spectra 29) and electronic structure theories with novel correlation effects (30,31,5). The electronic structure must explain the molecule s high stability, octahedral symmetry, and, most importantly, provide a simple picture of the bonding. At first glance, the traditional chemical models do not appear to be appropriate because sulfur seemingly forms six bonds to fluorines, yet the sulfur s2pA valence configuration allows for at most two covalent bonds. 

Sulfur hexafluoride (4.5) provides an example of a so-called hypervalent molecule, i.e. one in which the central atom appears to expand its octet of valence electrons. However, a valence bond picture of the bonding in SFg involving resonance structures such as 4.6 shows that the S atom obeys the octet rule. A set of resonance structures is needed to rationalize the observed equivalence of the six... 

Some molecules have six pairs of electrons around a central atom an example is sulfur hexafluoride (SFg), which has the Lewis structure... 

Elements in the third row of the periodic table and beyond often exhibit expanded octets of up to 12 (and occasionally 14) electrons. Consider the Lewis structures of arsenic pentafluoride and sulfur hexafluoride. 

Sulfur hexafluoride has six bonding pairs of electrons and no lone pairs. The equal repulsion between the electron pairs results in the structure shown in Figure 4.21. All F—S—F bond angles are 90°. We describe the shape as octahedral. 

The determination of the structures of the hexafluorides of sulfur, selenium and tellurium by the electron diffraction method. Proc. Natl. Acad. Sci. 19 (1933) 68—73. (L. O. Brockway and Linus Pauling). 

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