Tolman's rule

{{Short description|Rule describing chemical reactions}}

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.{{cite journal|last=Tolman|first=C.A.|title=The 16 and 18 electron rule in Organometallic Chemistry and Homogeneous Catalysis|journal=Chem. Soc. Rev.|year=1972|volume=1|issue=3|pages=337–353|doi=10.1039/CS9720100337}} 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.

:{{math|CH₃I + cis-[Rh(CO)₂I₂]⁻ → [(CH₃)Rh(CO)₂I₃]⁻}}

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

:{{math|Rh(PPh₃)₃ClH₂ → Rh(PPh₃)₃Cl + H₂}}