Elements of Bonding in Hypervalent Compounds

Another very important concept in this connection is that of FC. FC is the charge remaining on an atom when all the ligands have been removed homolytically. FCs are the charges that are commonly shown in structural formulas and reaction mechanisms. Thus the FC on nitrogen in NH4+ is -H and that on boron in BH4 is -1. 

Some of the consequences of this definition can seem a bit mind-bending, until you get used to them. Thus, applying this definition, we get an oxygen valence of 0 for OH and 4 for H3O+ After a while, these results won t seem quite as bizarre as they might do now. 

The main question we will try to address in this section concerns how the central atom in a hypervalent molecule accommodates more than eight valence electrons. For a long time, such expanded octets were thought to reflect participation by the d orbitals of the central atom. That explanation is now believed to be incorrect. Instead, a perfectly straightforward explanation is available from standard molecular orbital theory, which we will discuss below using the trigonal bipyramidal molecule PF5 as an example. 

Arguments of a similar nature also provide a qualitative explanation of the bonding in SFg, Brp3, IF5, IF7, and the various xenon fluorides. This book is not an appropriate place for a discussion of each of these cases. The key point is that, for the purposes of arrow pushing, all the element-fluorine bonds may be viewed as normal two-electron bonds. 

We are now in a position to discuss the question of multiple bonds between higher-valent main-group elements and typical multivalent ligands such as O and N. For this, consider the following molecules/ions  

---