Octet rule, deviation from

Deviations from the Octet Rule. Sometimes heavy atoms form so many covalent bonds as to surround themselves with more than four electron pairs. An example is phosphorus pentachloride, PCl in the molecule of this substance the phosphorus atom is surrounded by five chlorine atoms, with each of which it forms a covalent bond (with some ionic character). 

Other examples of molecules in which there is a deviation from the octet rule are the following ... 

The structures that are shown here are those in which the central atom makes use of the four orbitals corresponding to a noble-gas structure. There is evidence, however, that the central atom deviates from the octet rule, making use of additional orbitals and the unshared pairs of the oxygen atoms to form bonds with considerable double-bond character. Thus perchloric acid might be shown with the following structure, in which the chlorine atom forms double bonds with three oxygen atoms and a single bond with the fourth ... 

The less than unity tt electron populations in the same positions of nitrobenzene require resonance involving structures that deviate from the octet rule ... 

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