18electronsäännön

The 18-electron rule is a guideline used in organometallic chemistry to predict the stability of organometallic complexes. It states that stable transition metal complexes typically have a total of 18 valence electrons. These electrons are counted from the central metal atom and the ligands surrounding it. The 18-electron rule is analogous to the octet rule in main group chemistry, which predicts that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons.

To apply the 18-electron rule, one first determines the number of valence electrons contributed by the metal atom, which is usually equal to its group number in the periodic table. Then, the number of electrons contributed by each ligand is added. Ligands can donate electrons in different ways, such as neutral ligand donation (where the ligand's lone pairs are donated) or anionic ligand donation (where the ligand is considered to be anionic). For example, a carbonyl ligand (CO) is typically considered a neutral ligand donating two electrons.

When the sum of the metal's valence electrons and the electrons donated by all the ligands equals 18, the complex is generally considered to be stable. Deviations from the 18-electron rule can occur, particularly for early transition metals or in complexes with very bulky ligands. However, the rule remains a valuable predictive tool for understanding the bonding and reactivity of many organometallic compounds.