Hypervalency, definition

The main purpose of this chapter is to review the development of hypervalent chalcogen chemistry after the book of "Chemistry of Hypervalent Compounds edited by Akiba (1999).5 Although several types of compounds may belong to hypervalent ions or molecules by the definition,3a such hypervalent chalcogen compounds (E = S, Se, and Te) will be mainly discussed here that contain CT-type linear bonds.15... 

ICI3, and SeCLi were first reported almost two centuries ago, appropriate models to describe the bonding in these molecnles have only been developed recently. Whilst the term hypervalent is widely nsed and often misnsed there is currently no universally accepted definition of the word. Thus, it is instmctive briefly to explain the origin of the term before giving the working definition for this article. 

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. 

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... 

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