The Inert-Gas Shell Configuration Approach

The 18 electron-rule normally does not apply to transition metal complexes with 7i-donor ligands and metal atoms in normal oxidation states. In this case the metal orbitals with 7r-symmetry lie at relatively high energy and they stay empty or are only partially filled. In this way the formation of paramagnetic species can be possible. 

Metal Clusters. The 18-electron rule as an approach to the bonding in metal cluster supposes the skeletal atoms are held together by a network of localized bonds and that each individual cluster atom utilizes its nine atomic orbitals to form metal-metal bonds as well as to accommodate both metal valence electrons and ligand electron pairs. Consequently, in the application of the 18-electron rule to clusters, the following three assumptions should be taken into account  

Metal-metal bonds are localized on the polyhedron edges. 

All metal-metal bonds are two-center/two-electron bonds. 

Ligands are in general one electron pair donors irrespective of their charges. Cyclopentadiene (7i-Cp) and arenes are 5 and 6 electron donors respectively. 

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