New group 2 chemistry: A multiple barium-nitrogen bond in the CsNBa molecule

The existence of a series of triatomic molecules with the general formula MNM′, where M is an alkaline metal (K, Rb, Cs), and M′ is an alkaline earth metal (Ca, Sr, Ba), has been predicted by quantum chemical methods. Among these, the CsNBa molecule shows a feature not found before, the presence of a multiple bond between barium and nitrogen. As a consequence of this novel bonding situation, the molecule is linear. The same holds for all Ba triatomics, MNBa, independent of the nature of the alkali M atom, and for all Sr compounds, MNSr. The presence of a multiple bond makes CsNBa, and other related Ba and Sr molecules, particularly stable and appealing experimentally. The systems with the alkaline earth metal M′ = Ca, on the other hand, turned out to be bent. Calculations have also been performed on the negative ions BaN^- and CaN^-, which form a well-defined entity in the MNM′ systems (M′ = Ba, Ca). The results show that the two ions have a different electronic structure in the ground state, which is one reason for the different properties of the MNM′ systems and explains why the molecules containing the BaN^- moiety are linear, while those containing CaN^- are bent.