Past research already suggested heavy alkaline earth metals like these might break the rule because their d orbitals are often involved in bonding, something usually limited to transition metals.

The metal-insulator transition is one of the remarkable electrical properties of atomically thin molybdenum disulphide. Although the theory of electron–electron interactions has been used in ...

The degree of electron itinerancy and shell closing in the core-ionized state of transition metals probed by Auger-photoelectron coincidence spectroscopy.