Aquatic microbes convert inorganic mercury to methylmercury enzymatically by transferring a methyl group from a porphyrin-like cofactor called a corrinoid. A computational study now shows that such transfer occurs in a concerted fashion via a methyl radical, a mechanism influenced by relativistic effects in the mercury ion (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201606001). Relativistic effects arise in heavy elements from fast electron velocities and are characterized by altered orbital energies and coupling of electrons’ orbital and spin magnetic moments. In the case of mercury, the effect is to lower the energy of 6p orbitals to make them accessible for chemical bonding. A team led by Pawel M. Kozlowski of the University of Louisville studied methyl ligand exchange between a corrinoid and Hg(SCH3)2 by using density functional theory calculations, with and without taking relativistic effects into account. If the effects are ignored in the calculation, the mechanism seemingly occurs by stepwise transfer of CH3–. If the effects are accounted for, the results indicate that the reaction happens through a concerted transfer of CH3• in which the CH3•is partially transferred to the mercury while the departing SCH3 group is still weakly bound. Mixing of the mercury 6s and 6p orbitals allows the Hg to accommodate the three ligands.