Electron Counting and the Eighteen-Electron Rule
Electron counting combines a metal contribution with ligand donations. The eighteen-electron rule is a useful guide for many organometallic complexes, especially carbonyls, but other counts can also be stable. The count helps compare formulas and reaction steps; it does not prove stability.
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
The eighteen-electron rule is a guideline for many organometallic complexes, especially metal carbonyls. A metal supplies nine valence orbitals: five d, one s and three p. Suitable metal-ligand combinations can form a favorable eighteen-electron arrangement. These orbitals do not all belong to one outer shell. Their energies and bonding roles matter, so eighteen is not a universal stability test.
In neutral counting, start with the metal's group number. Each terminal CO contributes 2 electrons. Three terminal CO ligands therefore contribute 3 × 2 = 6 electrons, before adding the metal. For a charged complex, add one electron per negative charge or subtract one per positive charge. This charge adjustment belongs to the whole neutral count.
In ionic counting, assign ligand charges and find the metal oxidation state. Its d count is group number minus oxidation state. CO and phosphines donate 2 electrons in either method. Terminal halide, hydride and alkyl ligands donate 2 in ionic counting but 1 in neutral counting. Both methods must agree. Disagreement can reflect an incorrect charge, ligand classification, binding mode or arithmetic.
For a mononuclear complex whose only ligands are terminal CO, the neutral count is group number + 2n - q. Here n counts CO ligands and q is the signed complex charge: subtract positive charge and add electrons for negative charge. Neutral Mo in group 6 needs (18 - 6)/2 = 6 CO ligands, giving Mo(CO)6. Neutral Ni in group 10 needs 4 CO ligands, giving Ni(CO)4. These are eighteen-electron formula predictions, not stability proofs.
Many strong-field square-planar d8 complexes are stable at 16 electrons. The next orbital is strongly antibonding, so filling it need not help. Early-metal d0 complexes and some odd-electron species also depart from eighteen. Geometry or electron count alone does not prove stability. Adding an electron to a 17-electron complex makes 18 only if its ligands and binding modes stay unchanged.
Hapticity counts contiguous ligand atoms attached to a metal, not donated electrons. An eta2 alkene donates 2 in both methods. An eta3 allyl anion gives 4 ionically or 3 neutrally; eta5 Cp- gives 6 ionically or 5 neutrally. In a simple symmetric two-metal CO bridge, the 2-electron donation is shared as 1 per metal. A metal-metal single bond adds 1 to each metal. Use the stated model for other cluster bonding.
State charges and binding modes. Ionic and neutral counts must agree. Eighteen is a useful guideline, not proof of stability.
Original lesson, questions and flashcards on organometallic electron counting, written for this learning collection.
Three original readings on electron-counting conventions, the limits of the eighteen-electron rule and ligand binding modes, written for this learning collection.
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.