18 ELECTRON RULE

The 18 electron: A stable complex (with electron configuration of the next highest noble gas) is obtained when the sum of the metal d-electrons, electron donated from the ligand, and the overall charge of the complex equals 18

This was first proposed by N.V. Sigwick who extended the octet theory of G.N. Lewis and using it to explain coordination compounds. Ligands were considered Lewis bases and the metal ion in turn acted as a Lewis acid. The sum of all the electrons would assume a noble gas configuration of Kr, Xe or Rn.As with most rules of the thumb, the 18 electron rule is not always obeyed. 18 electrons are required to fill 5 d-orbitals, 1 s-orbital and 3 p-orbitals of a transition metal and hence reach the configuration of a noble gas.
Can hold 18 electron when full.

Therefore if we consider the following
Ni(CO)₄
Ni= 10
CO= 2
4CO= 4x2= 8
Therefore Ni + 4CO or 10 + 4 = 18
There are 2 main ways in which to count up the electrons.
1/ IONIC COUNTING: Ionic counting convention requires that ligands are anionic or cationic and that the oxidation state of the metal is appropriate for the realistic distribution of electrons.
2/ COVALENT COUNTING: This is an alternative & perhaps somewhat simpler approach. Covalent counting convention involves always treating the ligands as a neutral radical and the metal center as zerovalent.

Covalent counting is particularly useful if you which to determine odd electron complex fragment. This means predicting whether or not a complex has a metal-metal bond. Co₂(CO)₈ does this complex have a metal bond? ANSWER
Another useful method with the 18 electron rule is being able to predict various reactions.
Eg. Suppose we reacted Fe(CO)₅ with ethene or butadiene and we wished to determine the outcome of the reaction. Using the covalent counting method and the table of ligands we can effectively predict an answer.
Fe(0) 8e
5x CO 10e
18e
CH₂=CH₂ 2e
Therefore 1 CO would be displaced.
(C²H₄)Fe(CO)₄

or

CH₂=CH-CH=CH₂ 4e
Therefore 2 CO would be displaced.
Þ (C₄H₆)Fe(CO)₃

In order to effectively use the 18 electron rule we must have an idea as to what ligands donate how many electrons.

Ligand type	Ligand
1-electron ligands	-H, -Cl, -Br, -I, NR2, -CH3, -OR.
2-electron ligands
3-electron ligands
4-electron ligands
5-electron ligands
6-electron ligands
7-electron ligands
8-electron ligands

To summarise the 1st row transition elements favour 18-electron complexes, however complexes <18 can exist but mainly as reactive intermediates.
Second and Third row transition metals tend to form stable complexes smaller than 18. This is extremely important in the field of catalysts. 16-electron complexes are extremely common with nickel, palladium, and platinum.