Advanced Topic 698 of 1102 Inorganic Chemistry
Eighteen Electron Rule
Stable organometallic complexes tend to have 18 valence electrons around the metal.
In-depth explanation of Eighteen Electron Rule
The 18-electron rule is an empirical guide for transition metal complexes, which tend to be most stable with 18 valence electrons around the metal, filling its five d, one s and three p orbitals. It is analogous to the octet rule for main-group compounds. The count is the metal's valence electrons, minus its oxidation state for ionic complexes, plus the electrons donated by the ligands. For example, a carbonyl ligand donates two electrons and a cyclopentadienyl ligand donates five. Complexes with fewer electrons tend to be reactive and to bind extra ligands.
Going deeper: Eighteen Electron Rule
Key ideas: Stable organometallic complexes tend to have 18 valence electrons around the metal (filled s, p, d). Count: metal group electrons plus ligand electrons plus charge. CO and PR3 give 2, eta5-Cp gives 5 (neutral method), eta6-benzene 6, H and CH3 give 1 in the neutral method. Examples: Cr(CO)6: 6 + 12 = 18; Fe(CO)5: 8 + 10 = 18; Ni(CO)4: 10 + 8 = 18. Metal-metal bonds count one electron each.
Common mistakes and connections: Students apply it to all complexes; it works for strong-field pi-acceptor ligands with low oxidation states, not for typical Werner complexes of weak ligands. Square planar d8 complexes are stable with 16 electrons, as in Wilkinson catalyst. Counting must be consistent between ionic and neutral methods. Links to catalysis cycles, where steps swap between 16 and 18.
Visualization of Eighteen Electron Rule
Valence electron counts at the metal for common complexes: stable ones reach 18, whereas the 17-electron Mn(CO)5 radical dimerizes. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Iron pentacarbonyl, Fe(CO)5, has 8 electrons from iron plus 10 from five CO ligands, giving 18.
Frequently asked questions about Eighteen Electron Rule
What is Eighteen Electron Rule?
Stable organometallic complexes tend to have 18 valence electrons around the metal.
Is Eighteen Electron Rule a beginner, intermediate or advanced topic?
Eighteen Electron Rule is a advanced topic in Inorganic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Eighteen Electron Rule?
Study these earlier topics in Inorganic Chemistry first: Mond Process, Organometallic Compounds, Metal Carbonyls.
What comes after Eighteen Electron Rule?
The next topics in Inorganic Chemistry are Ferrocene and Zeigler-Natta Catalysts.
Can I practise Eighteen Electron Rule with quizzes?
Yes. The interactive lesson includes a self-check question on Eighteen Electron Rule, and the Inorganic Chemistry quiz draws on every topic in this area.
Where Eighteen Electron Rule fits in Inorganic Chemistry
Eighteen Electron Rule is topic 83 of 113 in Inorganic Chemistry.