An orbital explanation of electron-counting rules

There is a connection between an orbital description of electronic structure and the more elementary bonding discussions such as those reviewed in the Appendix. In this section we describe the connection of the 8- and 18-electron rules in order to provide a basis for understanding how the cluster electron-counting rules emerge from and are connected to molecular orbital descriptions of cluster bonding. 

1 The electron-counting rules as consequences of the closed-shell principle 

Exercise 1.5. What is an electron-counting rule derived from application of the closed-shell principle to H or He Why don t you find this rule in textbooks  

Answer. As both have a single valence function, n= 1 and the number of valence electrons lying in MOs in which a single H or He atom participate should be equal to two. A two-electron rule for two atoms seems unnecessary and counterproductive in the long run. 

3 Limitations of the 8- and 18-electron rule localized electron-deficient compounds 

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