Hyperconjugation—The Octet Rules

Note that in each of these resonance structures (and the many other similar ones that can be drawn) the sulfur atom always has an octet of electrons around it. The true structure is a composite of these equivalent resonance structures. The overall bonding is described as a combination of covalent and ionic contributions. The covalent contribution to the bonding involves four electron pairs spread out over the six sulfur-fluorine bonds. The ionic contribution to the bonding arises as follows. Note that for each Lewis structure two of the fluorines have -1 formal charges and the sulfur has a +2 formal charge, 

A similar treatment can be given for PCI5, using the following resonance structures, 

Thus the use of hyperconjugation preserves the octet rule and does not require the central atom to use d orbitals, which this model s proponents argue are too high in energy to participate in the bonding of these types of molecules. 

honest disagreements exist among chemists as to the best Lewis structures for molecules that, at least at first glance, appear to exceed the octet rule. This uncertainty shows the limitations of the Lewis model with its localized electron pairs. Note, however, that even with its limitations, it is still very useful because of its simplicity. The ability to obtain a reasonable bonding picture with a back-of-the-envelope model has led to the enduring influence of the Lewis model.  

Valence electrons assigned to each oxygen = 6 plus y(2) = 7 

---

Predict the molecular structure of KrF2. Using hyperconjugation, draw the Lewis structures for KrF2 that obey the octet rule. Show all resonance forms. 

---