Britannica Sources used in this answer

How does the 18-electron rule help explain the stability of some coordination compounds?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

The 18-electron rule, also known as the noble gas rule, provides a framework for understanding the stability of many coordination compounds, particularly those that are covalent in nature, such as metal carbonyls, cyanides, and organometallic compounds.123

According to this rule, coordination compounds are most stable when the total number of valence electrons—including those from the metal atom and the bonding electrons donated by the surrounding ligands—approaches but does not exceed 18. This number corresponds to the total number of electrons in the valence shells of the noble gases found in the same period as the metal.13

By applying this rule, chemists can predict the number of ligands a metal atom will coordinate with to achieve a stable electronic configuration. For example, stable mononuclear carbonyls like nickel tetracarbonyl, pentacarbonyliron, and hexacarbonylchromium all possess an 18-electron valence shell. While the rule is highly effective for covalent complexes, it is less applicable to more ionic, "outer-orbital" complexes, where configurations exceeding 18 electrons are common.123