N-X-L system

At this point, it is instructive to introduce a system to classify the bonding around an atom the N-X-L system. First suggested in 1980, it is particularly useful for the species discussed in this article. In this classification scheme, N is the number of valence-shell electrons formally assigned to the central atom X and L is the number of ligands directly bonded to X. The N-X-L designations are illustrated in Figure 2 for each species listed. 

A classification of hypervalent organo-nonmetallic derivatives, which have an electron-rich multicenter bonding system, has been devised by Perkins et al. in 1980. 2 This system, (the N-X-L system), describes the valence electron count and the coordination number. For an atom (X), the overall bonding system is described first by the number of valence shell electrons (N) formally directly associated with the atom (X), and by the number of ligands (L) directly bonded to it. A number of examples is shown below. 

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