The 18-electron rule is a guideline in organometallic chemistry that helps predict the stability of metal complexes, especially those involving transition metals.
🔹 Basic Idea
Just like main-group elements tend to follow the octet rule (8 electrons in the valence shell for stability), transition metals often achieve maximum stability when they have 18 valence electrons around them.
This number comes from:
- 2 electrons in the metal’s s orbital
- 6 electrons in the metal’s p orbitals
- 10 electrons in the metal’s d orbitals
→ Total = 18 electrons (a “noble gas–like” configuration for transition metals).
🔹 How It Works
- In organometallic complexes, the metal contributes some of its own valence electrons.
- The attached ligands donate the rest (through sigma bonds, π-backbonding, etc.).
- If the total electron count at the metal adds up to 18, the complex is usually thermodynamically stable.
🔹 Examples
- Ferrocene: Iron in ferrocene achieves 18 electrons, which explains its remarkable stability compared to many other organometallics.
- Metal carbonyls: Complexes like chromium hexacarbonyl or nickel tetracarbonyl obey the rule and are very stable.
🔹 Exceptions
- Not all complexes follow the 18-electron rule strictly.
- Early transition metals, bulky ligands, or special electronic effects may lead to stable complexes with fewer than 18 electrons.
- For example, many catalysts deliberately have fewer than 18 electrons to leave space for substrates to bind.