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18-Electron Rule

The 18 electron rule is a widely used principle in transition metal chemistry that plays a crucial role in determining the stability and reactivity of metal complexes.

This rule is based on the concept of achieving a filled electron shell, similar to the octet rule observed in main group elements. By following this rule, transition metal complexes can achieve greater stability and exhibit specific bonding patterns with ligands.

Significance of the 18-electron rule

The 18-electron rule is an essential concept in chemistry that provides a framework for understanding bonding in transition metal compounds. It helps predict the stability and behavior of metal complexes, guiding the design and synthesis of catalysts and materials.

Framework for understanding bonding

The 18-electron rule serves as a guide to understanding how transition metals form bonds with other elements.

According to this rule, transition metal complexes tend to be most stable when they have a total of 18 valence electrons surrounding the central metal atom. These valence electrons come from both the metal atom itself and the ligands attached to it.

This rule helps chemists determine how many ligands can bind to a transition metal atom, as well as their arrangement around it.

Predicting stability and behavior

By applying the 18-electron rule, chemists can make predictions about the stability and reactivity of transition metal complexes.

Compounds that adhere to this rule are typically more stable than those that do not. This knowledge allows scientists to select appropriate ligands for specific applications, ensuring optimal stability and performance.

Moreover, understanding the electron count in a complex helps predict its behavior in chemical reactions. The 18-electron rule aids in determining whether a complex will undergo substitution reactions or redox processes, providing valuable insights into reaction mechanisms.

Designing catalysts and materials

The 18-electron rule plays a crucial role in designing catalysts for various chemical transformations.

By considering electron counts and ligand properties, chemists can create transition metal complexes that exhibit high catalytic activity and selectivity.

This rule guides researchers in developing new materials with desirable properties. By carefully selecting ligands that fulfill the requirements of the 18-electron count, scientists can tailor material characteristics such as conductivity, magnetism, and optical properties.

18-electron rule for counting electrons

Counting electrons is an important concept in chemistry, especially. One commonly used method is the 18-electron rule, which helps determine the electron count for certain metal complexes. Here’s how it works:

Electrons from ligands and metals add to the total count.

In electron counting, both the ligands (atoms or molecules bonded to the central metal atom) and the metal itself contribute to the total electron count.

Each ligand contributes a set number of electrons based on its charge and bonding characteristics. The metal atom also contributes its valence electrons.

Lone pairs, single bonds, or multiple bonds

When counting electrons, lone pairs on atoms are counted as two electrons each. Single bonds between atoms are counted as two electrons as well. However, multiple bonds (such as double or triple bonds) between atoms are only counted as one electron each.

Get more stable by achieving 18 valence electrons.

The goal of electron counting using the 18-electron rule is to achieve a total of 18 valence electrons around the central metal atom.

This configuration often results in enhanced stability for many metal complexes. It mimics a noble gas configuration known as an “octet,” where there are eight valence electrons surrounding an atom.

To summarize:

  • Electron counting involves considering contributions from both ligands and metals.

  • Lone pairs and single bonds are counted as two electrons each.

  • Multiple bonds are counted as one electron each.

  • Achieving 18 valence electrons can enhance stability.

By applying these principles, chemists can better understand and predict the behavior of various metal complexes.

Application of the 18-electron rule in transition metals

Transition metals have a special affinity for forming stable complexes with 18 valence electrons. However, there are exceptions where fewer or more than 18 electrons can be favorable in certain cases.

Understanding electron counting based on the 18-electron rule is crucial as it aids in predicting complex structures and their stability.

Transition metal complexes and the 18-electron rule

Transition metal complexes are formed when transition metals, such as iron, copper, or nickel, interact with other molecules called ligands. These ligands can be neutral or charged species that donate electron pairs to form coordination bonds with the metal center.

The 18-electron rule states that most transition metal complexes tend to be stable when they have a total of 18 valence electrons surrounding the metal center. This stability arises from achieving a filled or half-filled d-orbital configuration, which is energetically favorable.

Exceptions to the 18-electron rule

While the 18-electron rule provides a useful guideline for understanding the stability of transition metal complexes, there are exceptions to this generalization. Some factors can lead to deviations from the ideal 18 electron count:

  1. Oxidation state: The oxidation state of the metal center can affect its ability to accommodate specific numbers of electrons.

  2. Accepting ligands: Certain ligands known as acceptor ligands can stabilize complexes with fewer than 18 electrons by accepting electron density from the metal.

  3. Class II compounds: In some cases, class II compounds violate the 18-electron rule due to steric effects or other electronic considerations.

Complex structures

Electron counting based on the 18-electron rule allows chemists to predict and rationalize complex structures observed in transition metal chemistry.

By determining how many valence electrons each ligand contributes and considering any charge present on the metal, chemists can determine the total electron count and predict the stability of a complex.

18-electron rule and complex stability

The 18-electron rule is a fundamental concept in chemistry that helps us understand the stability of transition metal complexes.

These complexes, which consist of a central metal atom or ion surrounded by ligands, play a crucial role in various chemical reactions and applications.

Complexes with 18 electrons are more stable because of electron pairing.

According to the 18-electron rule, complexes with exactly 18 valence electrons are considered highly stable. This stability arises from the phenomenon of electron pairing, where electrons occupy molecular orbitals in pairs.

When all available orbitals are filled and paired up, the complex achieves maximum stability.

Not following this rule can make things worse.

However, deviations from the 18-electron rule can lead to increased reactivity or instability of the complex. For example, if a complex has fewer than 18 valence electrons, it may seek to gain additional electrons through reactions with other species.

On the other hand, if a complex has more than 18 valence electrons, it may undergo rearrangements or reactions to reduce its electron count and achieve greater stability.

How Electron counting helps explain differences in complex properties

Electron counting plays a vital role in understanding and predicting the properties of transition metal complexes.

By knowing the number of valence electrons present in a complex, we can anticipate its reactivity, magnetic behavior, and even its color. Different types of complexes exhibit different electron counts based on their ligand environment and coordination geometry.


Ferrocene stable complex

One example that demonstrates the 18-electron rule in action is ferrocene. This compound consists of an iron atom sandwiched between two cyclopentadienyl rings.

The iron atom contributes 8 valence electrons, while each cyclopentadienyl ring donates 5 valence electrons, resulting in a total of 18 valence electrons. Due to this electron count, ferrocene is considered a stable complex.

Metal carbonyl complexes

Metal carbonyl complexes are another group of compounds that often adhere to the electron count dictated by the 18-electron rule. These complexes typically involve transition metals bonded to carbon monoxide ligands.

For example, tetracarbonylnickel (Ni(CO)4) and pentacarbonyliron (Fe(CO)5) both follow the 18-electron rule with their respective metal center surrounded by a specific number of CO ligands.

Wilkinson’s catalyst

Wilkinson’s catalyst, named after chemist Sir Geoffrey Wilkinson, is yet another notable example that showcases the application of the 18-electron rule. This complex consists of a rhodium atom bonded to three triphenylphosphine ligands and one chloride ion.

The rhodium center contributes 9 valence electrons, while each triphenylphosphine ligand donates 2 valence electrons and the chloride ion provides one electron. In total, Wilkinson’s catalyst adheres to the 18-electron rule with its configuration.


The 18-electron rule is a fundamental concept in transition metal chemistry that holds great significance. By understanding and applying this rule, chemists can gain valuable insights into the stability and reactivity of transition metal complexes.

The rule provides a framework for counting the number of valence electrons in these complexes, allowing researchers to predict their behavior and design new compounds with specific properties.


What happens if a complex violates the 18-electron rule?

When a complex violates the 18-electron rule by having fewer or more than 18 valence electrons, it often exhibits increased reactivity or instability. These deviations from the ideal electron count can lead to unusual bonding patterns or unfavorable electronic configurations.

However, it’s important to note that there are exceptions where complexes with fewer than 18 electrons can still be stable under certain conditions.

Can the 18-electron rule be applied to all transition metals?

The 18-electron rule is most commonly applied to transition metals from Groups 7-10 in the periodic table due to their typical valence electron counts. However, variations of this concept have been proposed for other groups as well.

It’s important to consider that while many transition metal complexes follow this guideline, there are exceptions and specific cases where the rule may not apply.

How does the 18-electron rule relate to catalysis?

The 18-electron rule plays a significant role in catalysis as it helps determine the stability and reactivity of transition metal catalysts. By following this guideline, chemists can design catalysts with optimal electron counts, ensuring efficient reactions.

The rule also aids in understanding reaction mechanisms and identifying key intermediates involved in catalytic processes.

Are there any limitations to the 18-electron rule?

While the 18-electron rule is a valuable tool in transition metal chemistry, it has its limitations. It is primarily based on observations and empirical evidence rather than strict theoretical principles.

Certain factors such as steric effects, ligand properties, and coordination geometry can influence complex stability and reactivity independently of electron count.

How can I apply the 18-electron rule in my research or studies?

To apply the 18-electron rule effectively, start by counting the number of valence electrons contributed by each ligand and metal center in a transition metal complex. Ensure that the total count adds up to approximately 18 electrons for stable complexes.

Understanding this concept will aid in predicting complex behavior, designing new compounds with desired properties, and analyzing reaction pathways accurately.