Experiment confirms relativistic bond weakening predicted by ADF in seaborgium

Relativistic effects can fundamentally change the chemical properties of superheavy elements, making their bonding behavior difficult to predict from trends in lighter elements. Seaborgium (Sg, element 106) provides a particularly interesting example, as its metal–carbon monoxide bonds were predicted to be weaker than those of tungsten, reversing the trend observed across the lighter group 6 elements.

In a recent Nature study, Yakushev and co-workers experimentally investigated the stability of seaborgium hexacarbonyl, Sg(CO)₆, using gas chromatography and measurements performed one atom at a time. They determined a first CO bond dissociation energy of 188 ± 9 kJ/mol, approximately 4 kJ/mol lower than for W(CO)₆. This unexpected trend agrees with earlier relativistic density functional theory calculations using ADF, which predicted that the Sg–CO bond would weaken rather than strengthen.

The earlier ADF calculations by Iliaš and Pershina showed that relativistic destabilization of seaborgium’s 6d orbitals weakens both CO-to-metal σ donation and metal-to-CO π back-donation, reducing the overall bond strength. In contrast, nonrelativistic calculations predicted a continued increase in bond strength from tungsten to seaborgium.

The study provides rare experimental evidence for relativistic effects on chemical bonding in superheavy elements and demonstrates how relativistic electronic-structure calculations with ADF can predict chemical behavior that is exceptionally difficult to investigate experimentally.