Tolman's rule
# Tolman's 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.

    $CH_3I$ + $cis-[Rh(CO)_2I_2]^−$ → $[(CH_3)Rh(CO)_2I_3]^−$

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

    $Rh(PPh_3)_3ClH_2$ → $Rh(PPh_3)_3Cl$ + $H_2$
