18 electron rule, a chemical thumb rule is used to predict the stability of a metal atom in an organometallic compound by evaluating whether it has 18 valence electrons. According to this rule, “In order for a transition element to be stable, it must have 18 electrons in its valence shell in the configuration of inert gas, and these electrons must form chemical bonds”. It is considered a simplified version of the EAN rule. EAN rule considers the total number of electrons of the atom, while the 18 electron rule only considers the number of valence electrons. The advantage of this rule is that applies to all rows of the periodic table.
18 electron rule is one of the valuable tools for predicting the formula of stable compounds as organometallic compounds, particularly those containing carbonyl and nitrosyl ligands, follow the rule with a high frequency.
18 Electron Rule:
18 electron rule was proposed by Irving Langmuir, an American scientist in order to explain the stability of transition metals and the organometallic compounds they form. The law is based on the fact that there are 2 s electrons, 6 p electrons, and 10 d electrons in the valence shell of transition elements, for a total of 18 electrons that can be either bonding or non-bonding. This corresponds to nine molecular orbitals that are either metal-ligand bonding or non-bonding when these nine atomic orbitals are combined with ligand orbitals.
There are two popular procedures for electron counting: neutral atom and oxidation state method. It is necessary to know how many electrons each ligand in a complex provides to the metal in order to apply either electron counting approach. The neutral atom method is generally considered to be of high preference as it doesn’t require the correct assignment of oxidation state.
According to this rule in metal carbonyls, the sum of the metal atom’s valence electrons and the electrons provided by the CO ligand is equal to 18. Carbonyl compounds that follow the 18 electron rule are stable, while those that do not follow it are unstable and hence do not exist. The number of outer electrons of the central atoms of the polynuclear metal carbonyls can be calculated by the following rules:
- For each metal atom, the number of valence electrons is added.
- In a metal-metal bond, the electron should be assigned homolytically between the two metals. As a result, each M-M bond serves as an electron donor.
- The terminal carbonyl group is a two-electron donor to a metal atom, while doubly bridging carbonyl or ketonic carbonyl groups give one electron.
- Because the number of outside electrons is unaffected by tautomerism, if two isomers (bridge and non-bridge) occur as a result of terminal bridging tautomerism of the carbonyl group, the total number of outer electrons in both cases should be 18.
Examples of 18 electron rule
EAN rule vs 18 electron rule
|EAN rule||18 electron rule|
|The total number of electrons (EAN) surrounding the coordinated metal ions is often equal to the atomic number of the inert gas that follows the central metal atoms in the periodic table. This is known as the effective atomic number rule or the Noble gas rule.||In order for a transition element to be stable, it must have 18 electrons in its valence shell in the configuration of inert gas, and these electrons must form chemical bonds. This rule is called the 18 electron rule.|
|Considers the total number of electrons of the atom.||Considers the number of valence electrons|
|Metals atom having a noble gas electron configuration is stable according to this rule.||Metals having 18 valence electrons is stable according to this rule.|
Applications of 18 electron rule
- In the determination of the total number of metal-metal bonds
- Used to Predict the stability of various organometallic compounds.
- To know the reactivity of transition elements.
- In the determination of the formula of organometallic compounds.
Limitations of 18 Electron Rule
Some of the major limitations of the 18 electron rules are:
- 16 electron compounds (both high spin octahedral and low spin square planar) fail to obey this rule.
- High spin compounds usually do not follow this rule as it lacks vacant orbitals in order to gain electrons in their valence shell.
- The metal complexes containing bulky ligands violate the 18 electron rule by interfering with the ligands for bonding.
- 𝛑-donating ligands do not follow the 18 electron rule.
- When an organometallic compound consists of more than 6 metal atoms, the 18 electron rule does not apply.
18 Electron Rule Video
- F.A. Cotton, G. Wilkinson, C.A. Murillo, and Manfred Bochmann Advanced Inorganic
Chemistry, (6th Edition), John Wiley and Sons, 1999.
- Huheey, James E, Ellen A. Keiter, and Richard L. Keiter. Inorganic Chemistry: Principles of Structure and Reactivity. New York, NY: HarperCollins College Publishers, 1993. Print.