Electronic configuration and the 18-electron rule

In this way, we rationalize the 18-electron rule that was previously discussed with reference to the valence electronic structure of the nearest nohle gas ( 1.1.2.1). 

The electrons that occupy the nonbonding MO are not used to form metal-Ugand bonds. They therefore correspond to the n electrons that remain on the metal in the classical counting scheme ( 1.1.2.3). The d notation for the electronic configuration of the metal assumes, however. 

There are also some complexes with more than 18 electrons, such as [Ni(H20)6] which possesses 20 electrons ( 1.1.2.1). Some antibond-ing MO must therefore be occupied, which can happen only if they are sufficiently low in energy as is the case in weak-field complexes. Organometallic complexes, characterized by the presence of one or several metal-carbon bonds, are strong-field complexes. It is therefore rare for them to possess more than 18 electrons. 

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