The 18-electron rule is fundamental to organometallic chemistry, providing a framework for understanding the stability and reactivity of transition metal complexes. It is analogous to the octet rule in organic and main group chemistry, but it applies to transition metals, which can accommodate up to 18 valence electrons in their s, p, and d orbitals. A complex that adheres to the 18-electron rule typically exhibits high stability because it achieves a "closed-shell" electronic configuration, minimizing its energy.
The importance of the 18-electron rule lies in its predictive power:
While the 18-electron rule provides a reliable guideline for many complexes, deviations occur due to steric or electronic factors. These exceptions are equally important, as they highlight the versatility and diversity of transition metal chemistry.
Electron counting is a fundamental tool in organometallic chemistry used to predict the stability, geometry, and reactivity of complexes. Transition metals, with their s, p, and d orbitals, can accommodate up to 18 valence electrons, analogous to the octet rule for main group elements. This is known as the 18-electron rule, which states that a complex is most stable when the metal has a full valence shell of 18 electrons.
The rule relies on counting electrons donated by the metal and its ligands. Stable complexes with 18 valence electrons often exhibit low reactivity, while those with fewer electrons (e.g., 16-electron complexes) tend to be more reactive and play important roles in catalysis.
The following steps outline a systematic approach to counting electrons in organometallic complexes:
Ligands donate electrons to the metal. The number of electrons donated depends on the type of ligand:
| Ligand | Electron Contribution |
|---|---|
| Neutral CO, PR₃ | 2 electrons |
| Anionic halides (Cl⁻) | 2 electrons |
| Cyclopentadienyl (Cp⁻) | 6 electrons |
| NO (linear, neutral) | 2 electrons |
| NO (bent, neutral) | 1 electron |
| Alkyls, hydrides (R⁻, H⁻) | 2 electrons |