eighteenelectron

Eighteenelectron is a term used in inorganic and organometallic chemistry to describe species that achieve an eighteen-electron configuration around a central metal atom or ion. The concept is most closely associated with the 18-electron rule, a heuristic that helps predict the stability and reactivity of many transition-metal complexes. When a metal center reaches 18 valence electrons through its own d-electrons plus electrons donated by ligands, the complex often assumes a closed-shell, noble-gas-like configuration that enhances stability.

Calculation of an eighteen-electron count involves determining the metal’s valence electrons and adding the electrons donated by ligands. A metal in a given oxidation state contributes its group number as valence electrons. Each L-type ligand donates two electrons, while X-type ligands donate one electron along with the metal-ligand bond. Summing these contributions yields the total valence electron count. Classic examples include Fe(CO)5 and Ni(CO)4, both commonly cited as 18-electron species: the metal provides its d-electrons, and the carbonyl ligands donate the remaining electrons to reach 18.

The eighteen-electron rule is a useful guide but not universal. While many low-oxidation-state, carbonyl-rich complexes conform to 18 electrons, numerous stable species fall outside this count, especially among early transition metals, high-coordinate compounds, multiple metal–metal bonds, or clusters. As a heuristic, eighteenelectron helps chemists rationalize bonding, predict stability, and anticipate reactivity, but exceptions are well-documented in the literature.

See also: eighteen-electron rule, electron counting, organometallic chemistry.