18-electron rule

The 18-electron rule is a heuristic that many stable transition-metal complexes have 18 valence electrons around the metal, filling its valence shell.

Valence electron counting: A method for tallying electrons assigned to atoms or molecules in chemical structures. It provides the bookkeeping framework for determining whether a complex reaches 18 electrons.
Transition metal: An element whose atom or common ions have an incomplete d subshell. The rule is chiefly applied to complexes containing these metals.
Ferrocene: An organometallic compound with an iron atom between two parallel cyclopentadienyl rings. Its iron center reaches 18 electrons through donation from both aromatic rings.
16-electron rule: A heuristic that many stable square-planar transition-metal complexes have 16 valence electrons. It describes common stable complexes that stop short of the 18-electron total.
Oxidation state: A formal charge assigned to an atom under the assumption that bonding electrons belong to the more electronegative atom. The ionic counting method uses the metal’s oxidation state to establish its d-electron count.
d orbital: An atomic orbital with angular momentum quantum number two, found in sets of five in a d subshell. The metal’s d orbitals supply the central part of its valence-electron count.
Nickel tetracarbonyl: A volatile compound with a nickel atom bonded to four carbon monoxide ligands. It is a classic 18-electron complex, with each carbon monoxide donating two electrons.
18-electron complex: A transition-metal complex with a formal count of 18 valence electrons at its metal center. This is the favored count in the rule’s typical cases, not a guarantee of stability.
Ligand: An ion or molecule that donates electron density to a central atom in a coordination complex. Ligand donor electrons add to the metal’s count and determine whether the rule is met.
Coordination complex: A compound or ion containing a central metal atom or ion bonded to surrounding ligands. The 18-electron count is applied to the metal center within these structures.

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