--- title: "Electron Counting and the Eighteen-Electron Rule" description: "Organometallic compounds are stable when the metal's valence shell is full, which for a transition metal means eighteen electrons. Counting them is a bookkeeping procedure that predicts which formulas" canonical: https://lightmysky.com/learn/science/electron-counting-and-the-eighteen-electron-rule-mt_uab5c5pODY source: https://lightmysky.com/learn/science/electron-counting-and-the-eighteen-electron-rule-mt_uab5c5pODY.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Electron Counting and the Eighteen-Electron Rule Organometallic compounds are stable when the metal's valence shell is full, which for a transition metal means eighteen electrons. Counting them is a bookkeeping procedure that predicts which formulas exist and which reactions are possible. Subject: Science · Area: Chemistry · Ages 20 to 21 Page: https://lightmysky.com/learn/science/electron-counting-and-the-eighteen-electron-rule-mt_uab5c5pODY ## Ready when they can - Counts electrons by both the ionic and the neutral method and gets the same total - Assigns the contribution of common ligands, including bridging and multi-hapto ones - Predicts how many carbonyl ligands a given metal will carry in a neutral complex - Names classes of complex that break the rule and explains why square planar cases stop at sixteen ## Lesson: Counting to eighteen at a metal centre Transition metals play a bigger version of the octet idea. Their outer shell holds nine orbitals, one s plus five d plus three p, which is room for eighteen electrons. A metal complex is usually extra stable when the electron count at the metal adds up to eighteen. **Example.** Counting is simple bookkeeping. Each ligand donates a fixed pile: a carbonyl donates 2. The metal adds its own valence electrons, read from its group number: chromium in group 6 gives 6, iron in group 8 gives 8, nickel in group 10 gives 10. Chromium with six carbonyls lands on 6 plus 12, which is 18. Count neutral or as an ion; both routes must agree. If they disagree, one pile was miscounted. Use the count to predict formulas: pair the metal group number with 2 electron donors such as carbonyls and see how many reach eighteen. Chromium in group 6 needs twelve more, which is six carbonyls, and nickel in group 10 needs eight more, which is four carbonyls. A seventeen count sits one short; gaining one electron by reduction is one route to eighteen. Note the exceptions: square planar complexes often stop at sixteen, and some classes break the rule for their own reasons. **Recap.** Add the metal group number to the ligand donations, check both counting routes agree, and expect eighteen. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Stability Constants and the Kinetics of Ligand Substitution](https://lightmysky.com/learn/science/stability-constants-and-the-kinetics-of-ligand-substitution-mt_XiuL697fut) ## Opens up - [Organometallic Ligands: Carbonyls, Phosphines and Hapticity](https://lightmysky.com/learn/science/organometallic-ligands-carbonyls-phosphines-and-hapticity-mt_JXYSQDYlJ-)