Mercury Methylation by Cobalt Corrinoids: Relativistic Effects Dictate the Reaction Mechanism.

The methylation of Hg(II) (SCH3 )2 by corrinoid-based methyl donors proceeds in a concerted manner through a single transition state by transfer of a methyl radical, in contrast to previously proposed reaction mechanisms. This reaction mechanism is a consequence of relativistic effects that lower the energies of the mercury 6p1/2 and 6p3/2 orbitals, making them energetically accessible for chemical bonding. In the absence of spin-orbit coupling, the predicted reaction mechanism is qualitatively different. This is the first example of relativity being decisive for the nature of an observed enzymatic reaction mechanism.