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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 ...
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 ).
Tolman's rule states that, in a certain chemical reaction, the steps involve exclusively intermediates of 18- and 16 electron configuration. The rule is an extension of the 18-electron rule . This rule was proposed by American chemist Chadwick A. Tolman . [ 1 ]
Invoking this ligand-only orbital allows for satisfaction of the 18-electron rule in M(CO) 8 complexes, and is stabilized by the field effect of the metal on the ligand cage. [14] Alkaline earth metals are capable of adding their two valence electrons to the degenerate (n-1)d orbitals of e g symmetry.
These complexes are isoelectronic and, incidentally, both obey the 18-electron rule. The formal description of nitric oxide as NO + does not match certain measureable and calculated properties. In an alternative description, nitric oxide serves as a 3-electron donor, and the metal-nitrogen interaction is a triple bond. linear and bent M-NO bonds
The Fe-NO bonds are linear indicating NO is acting as a three electron donor. [3] The diamagnetic compound obeys the 18-electron rule . The dark red colour of the complex is attributed to a number of charge-transfer interactions between the iron core and nitrosyl ligands.
The two other principal iron carbonyls are Fe 3 (CO) 12 and Fe 2 (CO) 9. Group 9 elements with 9 valence electrons and are expected to form tetracarbonyl dimers M 2 (CO) 8. In fact the cobalt derivative of this octacarbonyl is the only stable member, but all three tetramers are well known: Co 4 (CO) 12, Rh 4 (CO) 12, Rh 6 (CO) 16, and Ir 4 (CO) 12.
In chemistry the polyhedral skeletal electron pair theory (PSEPT) provides electron counting rules useful for predicting the structures of clusters such as borane and carborane clusters. The electron counting rules were originally formulated by Kenneth Wade , [ 1 ] and were further developed by others including Michael Mingos ; [ 2 ] they are ...