18-Electron rule

The 18- electron rule in chemistry states that complexes with 18 valence electrons are particularly stable. The rule can be applied to many complexes of the transition metals. It results from the molecular orbital theory and applies to elements from the fourth period of the periodic table.

Examples

The transition metal chromium compound is chromium hexacarbonyl Cr (CO ) 6, the iron -stable iron pentacarbonyl Fe (CO) 5, while the nickel is particularly stable nickel tetracarbonyl Ni ( CO) 4. In all three cases, the central atom (oxidation state = 0), then 18, and thus the bonding electron configuration of a noble gas krypton: chromium itself has six valence, iron and nickel eight ten. Since each molecule of CO contributes to the binding of two electrons, 4, eight valence electrons have to be included as in Cr (CO) 6 twelve, for Fe (CO) 5 and ten Ni (CO).

Limitations of the model

With the 18- electron rule, for example, the stability of ferrocene (18 electrons) and the reducing nature of the metallocene can cobaltocene (19 electrons) and nickelocene (20 electrons) are explained. Nickelocene is less reactive because there are 2 electrons in a weakly antibonding orbital.

In the early transition metals, the 18-electron rule is often not satisfied for steric reasons. This means that not enough space around the central particle is present to allow sufficient ligands - and thus missing electrons - to attach. For example, has Vanadiumhexacarbonyl V ( CO) 6, only 17 electrons on the vanadium atom. A possible way out of this electron deficiency would be the dimerization to form a covalent V- V binding ( gain of a common electron). This reaction is no longer possible for reasons of space. However, V (CO) 6 acts as a moderately strong oxidizing agent, there by taking an electron into the anion [V (CO ) 6] - 18 - is converted to electrons.

Another reason for non- compliance with the rule is the presence of more electrostatic ( electro- valent, ionic ) binding relationships. Here are orbital overlaps crucial - it must also be no rules that originate from the orbital theory, are followed - but the binding is based primarily on classical electrostatics. As a typical example of the known stable Kupfertetrammin complex [ Cu (NH3 ) 4] 2 was singled out, which according to the usual method of counting 17 valence electrons ( Cu2 : 9 e - NH3 4 x 4 x 2 should have e-).

Also, just the mentioned complexes of the early transition metals are to be mentioned under this point of view again. These metals from the transition metals, the smallest electronegativity and form so many complex ligands rather electrostatically bound complexes that do not satisfy the 18-electron rule ( must ). The same is true for complexes of alkali and alkaline earth metals - they must also satisfy neither the 8 -electron nor the 18-electron rule, because of covalent bonding are negligible here.

9768
de