text: Tolmans 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, Tolmans 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: