Tolman's rule

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.^[1] However, there are exceptions to Tolman's rule, even for reactions that proceed via 2e^− 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.

CH₃I + cis-[Rh(CO)₂I₂]^− → [(CH₃)Rh(CO)₂I₃]^−

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

Rh(PPh₃)₃ClH₂ → Rh(PPh₃)₃Cl + H₂