Electron transfer as a probe for relativistic effects in the ionization of atoms: Onset of electron transfer from benzene to heavy Group 11 and 12 transition metal cations

•Used ICP/SIFT to react Group 11 and 12 atomic cations with C6H6, C6F6 and CS2.•Lighter ions in these two triads become ligated with high efficiency.•Electron transfer emerges dramatically with the heavier Au+ and Hg+.•Due to relativistic effects in IE(Au, Hg) that make it exothermic.•As predicted by quantum chemical calculations of 1s orbital energies.