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.
What a learner can do afterwards
- 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
1 · Read
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.
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.
Add the metal group number to the ligand donations, check both counting routes agree, and expect eighteen.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.