Observation of relativistic bond weakening in seaborgium hexacarbonyl

The seventh row of the periodic table of the elements has been completed with the discovery of the five heaviest elements from flerovium (Fl, Z = 114) through oganesson (Og, Z = 118)¹. Chemical properties of the superheavy elements (SHE, Z ≥ 104) are unique owing to the strong influence of relativistic effects on their valence electron shells². The stability of the metal-ligand bonding in carbonyl complexes is a property that is predicted to be affected by such effects^3,4. Here we present an experimental gas chromatography study on the stability of the Sg(CO)₆ complex. The measured formation yields of Sg(CO)₆ and W(CO)₆ enabled determining the first bond dissociation energy (FBDE) of Sg(CO)₆, which is 4(2) kJ mol^-1 lower than that of W(CO)₆ and corresponds to a value of 188(9) kJ mol^-1. Thus, the trend of increasing FBDE values of hexacarbonyls within group 6 of the periodic table is reversed from W to Sg. Our result agrees with state-of-the-art theoretical calculations, which predict a decrease in the FBDE from W(CO)₆ to Sg(CO)₆ owing to the relativistic destabilization of the 6d atomic orbitals (AOs) in Sg (ref. ⁴). Also, the adsorption enthalpy value of Sg(CO)₆ on gold of ${45}_{-6}^{+4}\mathrm{kJ}{\mathrm{mol}}^{-1}$ is reported.