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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 ] However, there are exceptions to Tolman's rule, even for reactions that proceed via 2e − steps, because many reactions involve intermediates with fewer than 16 electrons.
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,
One consequence of their increased size, these clusters often contain atoms at their centers. A prominent example is [Fe 6 C(CO) 16] 2-. In such cases, the rules of electron counting assume that the interstitial atom contributes all valence electrons to cluster bonding. In this way, [Fe 6 C(CO) 16] 2-is equivalent to [Fe 6 (CO) 16] 6-or [Fe 6 ...
From the perspective of the 18-electron rule, the four ligands each provides two electrons, for a total of 16-electrons. As such the compound is coordinatively unsaturated , i.e. susceptible to binding substrates (alkenes and H 2 ).
The number of ligands that react with a central metal atom can be found using the 18-electron rule, saying that the valence shells of a transition metal will collectively accommodate 18 electrons, whereas the symmetry of the resulting complex can be predicted with the crystal field theory and ligand field theory.
The exception is square-planar because square-planar complexes typically abide by the 16-electron rule. Assuming ligands act as two-electron donors the metal center in square-planar molecules is d 8. To relate an octahedral fragment, ML n, where M has a d x electron configuration to a square planar analogous fragment, the formula ML n−2 where ...
Quantum chemical calculations using density functional theory confirm that Ca, Sr, and Ba can indeed utilize their (n-1)d in bonding to satisfy the 18-electron rule. [1] [6] These computational results support the hypothesis that alkaline earth octacarbonyl complexes follow the 18-electron rule and are comparable to carbonyl transition metal ...