Isosteric and Isoelectronic Relationships

The electron-pair/octet rule formalism shows significant limitations when applied to more complex molecules, because alternative isomeric structures, all of which are consistent with the Lewis assumptions, may be written. For example, molecules and ions with four atoms and a total of 22 valence electrons are consistent with all the Lewis structures shown in Fig. 10. Linear, angular, ring and butterfly structures 

The less electronegative atom is generally located in central locations and the more electronegative atoms on the outside. 

The heavier elements favour ring structures rather linear structures with multiple bonds. 

Structures with uncharged atoms are generally preferred relative to charged structures, and positively charged atoms are generally disfavoured for very electronegative atoms such as F and O. 

Structures with the same charge on adjacent atoms are disfavoured. 

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