Hypervalent species, definition

The definition of hypervalent species as ions or molecules of the elements of Groups 15—18 bearing more than eight electrons within a valence shell was proposed byj. I. Musherin 1969 (1969AGE54). General aspects of bonding... 

If hypervalent literally means exceeding the lowest chemical valence, then even NH4+ is hypervalent, despite the fact that the Lewis-Langmuir theory easily accounts for its stability. Furthermore, if the definition of hypervalency is restricted to Groups 15-18, then this artificially excludes species such as SiFs and SiFe, which are isoelectronic and isostructural with PF5 and SFe, respectively. The ambiguities in the original definition have led to several changes. Today, no one would regard NH4+ as hypervalent, while those that consider PF5 and SFe hypervalent would classify SiFs" and SiFe in the same way. 

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