The 18-electron rule is a guideline in organometallic chemistry that helps predict the stability of transition metal complexes.
A transition metal complex tends to be most stable when the total number of electrons around the metal center (its own valence electrons + those donated by ligands) adds up to 18 electrons.
Why 18?
- Transition metals have valence orbitals made up of:
- 1 s orbital (can hold 2 electrons)
- 3 p orbitals (can hold 6 electrons)
- 5 d orbitals (can hold 10 electrons)
- Together, these orbitals can hold 18 electrons maximum.
- When the orbitals are filled (like in a noble gas configuration), the complex achieves extra stability.
Examples
- Fe(CO)₅ (iron pentacarbonyl) → follows the 18-electron rule and is very stable.
- Ni(CO)₄ → also follows the 18-electron rule.
Limitations
- Not all complexes follow it strictly.
- Complexes with bulky ligands, electron-poor metals, or metals in higher periods often deviate.
- Still, it’s a useful rule of thumb for predicting stability in organometallics.