16 and 18 electron rule

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. 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⁻.

Many examples of homogeneous catalysis involving organometallic complexes involve shuttling of complexes between 16 and 18 electron configurations. 16-electron complexes often form adducts with Lewis bases and, if low-valent, undergo oxidative addition.

Conversely, complexes of 18 electron configuration tend to dissociate ligands or undergo reductive elimination:

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