Nitrogen Pentafluoride, the Octet Rule, and Hypervalency

hewavs. Modeling Marvels, DOI 10.1007/978-1 020-6973-4.4, Springer Science+Business Media B.V. 2008 

Did chemists regard as sacrosanct the stricture against the presence of more than eight electrons in the valence shell of the atoms lithium to neon Certainly in 1923 Lewis did not so regard it When I claimed that nitrogen is never more than quadrivalent I did not mean to deity the theoretical possibility of a compound such as NF5...What I maintained...is that in aity compound now known the nitrogen atom is never attached to more than four bonds. (p. Ill of [5]). We shall see that Lewis crafted better than he knew even for NF5, if it can be made, the octet mle will evidently not be violated ( ), which brings us to the term hypervalency. 

A hypervalent compound is typically considered to be one in which the central atom has in its valence shell more than eight valence electrons, as in NF5 or the known PF5 and SFe, which appear to have 10, 10, and 12 electrons, respectively, in their valence shells [10]. The valence of an atom may reasonably be taken as the number of electrons it uses in bonding, as expressed in a Lewis stmcture. Thus carbon in CH2, CH3 and CH4 is taken, nncontroversially, to be di-, tri- and tetrava-lent, respectively. Nitrogen in NF3 is trivalent, and in NF5 would be pentavalent, according to simple Lewis stmctures  

Expanding the octet with d atomic orbitals. For elements of the second full period, like silicon and sulfur, the formation of pentacoordinate and hexacoordinate species 

A N atom with 5 valence electrons, one 2s, and three p orbitals 

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