Relativistic Effects in the Ligation of Atomic Coinage Metal Cations with O₂ and C₆H₆: Anomalous Formation of Relativistic Mono- and Bis-adducts with Au

The interaction of the atomic coinage metal cations Cu⁺, Ag⁺, and Au⁺ with O₂, a weak ligand, and C₆H₆, a strong ligand, was investigated with measurements of rate coefficients of ligation and quantum-chemical computations of ligation energies with an eye on relativistic effects going down the periodic table. Strong "third row enhancements" were observed for both the rate coefficients of ligation and ligation energies with the O₂ ligand and for the formation of both the mono- and bis-adducts of M⁺ and the monoadduct of M⁺(C₆H₆). The computations revealed that the third-row enhancement in the ligation energy is attributable to a relativistic increase in the ligation energy. This means that rate coefficient measurements down the periodic table for the ligation of coinage metal cations with O₂ provide a probe of the relativistic effect in ligation reactions, as expected from the known dependence of the rate coefficient of ligation on the ligation energy. The much stronger benzene ligand was observed to ligate the atomic coinage metal cations with nearly 100% efficiency so that there is no, or only slightly, visible third-row enhancement despite the strong relativistic effect in the binding energy that is revealed by the calculations. Relativistic effects contribute substantially to the extraordinary stability against deligation of all the observed mono- and bis-adducts of Au⁺ relative to Ag⁺, truly a "third-row enhancement".