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

See also

References

  1. Tolman, C.A. (1972). "The 16 and 18 electron rule in Organometallic Chemistry and Homogeneous Catalysis". Chem. Soc. Rev. 1 (3): 337–353. doi:10.1039/CS9720100337.
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