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The 18-electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes, especially organometallic compounds. [1] The rule is based on the fact that the valence orbitals in the electron configuration of transition metals consist of five ( n −1)d orbitals, one n s orbital ...
The rule is an extension of the 18-electron rule. This rule was proposed by American chemist Chadwick A. Tolman. [1] As stated above, Tolman's rule, even for reactions that proceed via 2e − steps, is incorrect because many reactions involve configurations of fewer than 16 e −.
The high oxidation state stabilizes the highly reduced ligands. The low d electron count allow for many bonds between ligands and the metal center. A d 0 metal center can accommodate up to 9 bonds without violating the 18 electron rule, whereas a d 6 species can only accommodate 6 bonds.
Many rules in chemistry rely on electron-counting: Octet rule is used with Lewis structures for main group elements, especially the lighter ones such as carbon, nitrogen, and oxygen, 18-electron rule [2] in inorganic chemistry and organometallic chemistry of transition metals, Hückel's rule for the π-electrons of aromatic compounds,
In organometallic chemistry, the Green–Davies–Mingos rules predict the regiochemistry for nucleophilic addition to 18-electron metal complexes containing multiple unsaturated ligands. [1] The rules were published in 1978 by organometallic chemists Stephen G. Davies, Malcolm Green, and Michael Mingos.
The complete filling of these nine lowest-energy orbitals with electrons, whether those electrons originate from the metal or from any ligands, is the basis of the 18-electron rule. If it is correct, perhaps it should be included in the article.
The 18-electron rule is helpful in predicting the stabilities of organometallic complexes, for example metal carbonyls and metal hydrides. The 18e rule has two representative electron counting models, ionic and neutral (also known as covalent) ligand models, respectively. [7] The hapticity of a metal-ligand complex, can influence the electron ...
As is the case for many other η 1-allyl complexes, the monohapticity of the allyl ligand in this species is enforced by the 18-electron rule, since CpFe(CO) 2 (η 1-C 3 H 5) is already an 18-electron complex, while an η 3-allyl ligand would result in an electron count of 20 and violate the 18-electron rule.