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18 Electron Rule

LEARN MORE ABOUT-   18 ELECTRON RULE 




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ligand with electron  donation 
LigandElectrons contributed
(neutral counting)
Electrons contributed
(ionic counting)
Ionic quivalent
X12X; X = F, Cl, Br, I
H12H
H10H+
O24O2−
N36N3−
NR322NR3; R = H, alkyl, aryl
CR224CR2−
2
Ethylene22C2H4
cyclopentadienyl56C
5
H
5
benzene66C6H6
The 18-electron rule is a rule used primarily for predicting and rationalizing formulae for stable metal complexes, especially organometallic compounds. The rule is based on the fact that the valence shells of transition metals consist of nine valence orbitals (one s orbital, three p orbitals and five d orbitals), which collectively can accommodate 18 electrons as either bonding or nonbonding electron pairs. This means that, the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period. The rule and its exceptions are similar to the application of the octet rule to main group elements. The rule is not helpful for complexes of metals that are not transition metals, and interesting or useful transition metal complexes will violate the rule because of the consequences deviating from the rule bears on reactivity. The rule was first proposed by American chemist Irving Langmuir in 1921.



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DESH DEEPAK A P S CHAUHAN

M.Sc. CHEMISTRY!

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