The
18-electron rule is a rule used primarily for predicting and rationalizing formulae for stable metal complexes, especially
organometallic compounds. The rule is based on the fact that the
valence shells of
transition metals consist of nine valence orbitals (one
s orbital, three
p orbitals and five
d orbitals), which collectively can accommodate 18
electrons as either bonding or nonbonding electron pairs. This means that, the combination of these nine
atomic orbitals with
ligand orbitals creates nine
molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the
noble gas in the period. The rule and its exceptions are similar to the application of the
octet rule to main group elements. The rule is not helpful for complexes of metals that are not transition metals, and
interesting or useful transition metal complexes will violate the rule because of the consequences deviating from the rule bears on reactivity. The rule was first proposed by American chemist
Irving Langmuir in 1921.