Tolman's rule states that, in certain chemical reactions, 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. However, there are exceptions to Tolman's rule, even for reactions that proceed via 2e<sup>âÂÂ</sup> steps, because many reactions involve intermediates with fewer than 16 electrons.
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: