Hybridization and Covalency in the Group 2 and Group 12 Metal Cation/Rare Gas Complexes

We provide a consistent set of interaction energy curves for the group 2 (IIA) and group 12 (IIB) metal cation/rare gas complexes, M+–RG, where M+ = Be+–Ra+ and Zn+–Hg+ and RG = He–Rn. We report spectroscopic constants derived from these, compare them with available data, and discuss trends in the values. We gain insight into the interactions that occur using a range of approaches: reduced potential energy curves; charge and population analyses; molecular orbital diagrams and contour plots; and Birge–Sponer plots. Although sp hybridization occurs in the Be+–RG, Mg+–RG and group 12 M+–RG complexes, this appears to be minimal and covalency is the main aspect of the interaction. However, major sd hybridization occurs in the heavier group 2 M+–RG systems, which increases their interaction energies but there is minimal covalency. Examination of Birge–Sponer plots reveals significant curvature in many cases, which we ascribe to the changing amounts of hybridization or covalency as a function of internuclear separation. This suggests why the use of a simple electrostatics-based model potential to describe the interactions is inadequate.