Abstract
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.
Highlights
Interactions between metal cations and rare gas atoms are the simplest systems with which to investigate molecular interactions that can be viewed as underpinning catalysis as well as many bioinorganic processes
In a number of cases it was evident that the systems were showing behaviour that was reminiscent of incipient chemical bonding, and so various methods were employed to establish the extent to which “chemistry” was present in the interactions, as opposed to just “physics”. (Here, the use of the term “chemistry” includes covalency and hybridization that occurs as a result of the interaction.) Initially, this was undertaken using a physical “model potential” 18 including damping,[1,6] but we have since employed various population analyses, as well as molecular orbital diagrams including contour plots
The previous IECs from our group for the titular species were used to calculate ion transport coefficients and this provided another avenue of comparison by which to evaluate the quality of the calculated potentials via ion transport studies of metal cations moving through a bath of rare gas.[7,8,9,12,13]
Summary
Interactions between metal cations and rare gas atoms are the simplest systems with which to investigate molecular interactions that can be viewed as underpinning catalysis as well as many bioinorganic processes. As previous investigations have highlighted, interactions in the Group 11 M+-RG complexes exhibit rather strong binding energies,[10] with Au+-Xe in particular being very strongly bound and proposed as exhibiting a degree of covalency.[11,19] the Group 1 complexes are comparatively weakly bound and were found to be described well by a ‘physical’ model potential that included the main induction and dispersion terms together with a simple repulsion potential.[1,6] Group 13 is a different case again, with the heavier members of the M+-RG group showing similar behaviour to the Group 1 complexes,[16] and the lightest members, involving B+,17 showing strong binding energies with some incipient covalency Another logical comparison presents itself in the case of the Group 2 and Group 12 M+–RG complexes. We undertake analyses of the interactions across the series using various approaches, which allow us to compare and contrast the interactions in these valence isoelectronic species
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