The effects of dispersive Cn/Rn-attraction on M+/Rg bonding (M+=atomic metal ion, Rg=rare gas atom)

Abstract

It has been shown, using a “model-potential” analysis, that −Cn/Rn dispersive terms can be an important part of the physical bonding in M+/Rg complexes (M+=atomic metal ion, Rg=rare gas atom) for M+ ions with large, polarizable outer-shell electron clouds. The model potential equation consists of all attractive terms (accurately calculated or estimated) out to 1/R8, as well as an Ae−bR repulsive term. From known De, Re, and ωe values, and the first and second derivatives of the model potential, the repulsive constants A and b as well as the effective charge Z of M+ in a particular M+⋅Rg electronic state, can be determined. For the typical M+⋅Rg states considered here, Z=1.02±0.07, indicating that no extra “chemical” effects are necessary to explain M+/Rg bonding. Furthermore, the trends in the derived Ae−bR repulsive curves make good qualitative sense. A term-by-term analysis for M+⋅Rg states where the M+ ion is small and unpolarizable [such as Na+(2p6)⋅Rg] shows that −Cn/Rn terms contribute only a few percent to the bond strengths, while for M+⋅Rg states where M+ is large and polarizable [for example, Mg+(3s)⋅Rg], the −CnRn terms can contribute on the order of 40%–50% to the bond strengths, thus rationalizing semiquantitatively several heretofore puzzling De, Re, ωe comparative values.

Analysis of the bonding in alkali-cation/Rg complexes (Rg=He–Xe) using a simple model potential

“…We analyze this data using the ''long-range-forces'' model potential proposed by Breckenridge and co-workers [1][2][3]. All the attractive terms out to R À8 are included in the potential, and the usual BornMayer repulsive term of the form Ae ÀbR is used (and commented on below).…”

“…The original version of the Breckenridge model potential [1,3] does not contain so-called damping functions, which are meant to allow for damping of the purely long-range attractive forces at shorter ranges, where the electron clouds of the M + ion and the Rg atom interpenetrate. Breckenridge and co-workers [1] intentionally made no attempt to include such damping functions, both because of simplicity in comparing many M + /Rg diatomic complexes, and because there has in the past been some discussion regarding the correct mathematical form of such functions [15].…”

“…There has been controversy in the past about whether some M + /Rg interactions, especially those of transition-metal ions with Kr and Xe, can be described by purely electrostatic and dispersive attraction (''physical'' bonding) or whether there is an extra component due to Lewis acid/base covalent interactions (''chemical'' bonding). Breckenridge and co-workers [1][2][3] in 2001 showed that, using an entirely ''physical'' model potential, that almost all cases of M + /Rg bonding could be rationalized qualitatively by ''physical'' interactions, with no extra ''chemical'' bonding required [1]. (Possible exceptions were Au + /Kr and Au + /Xe, which will be treated by us in a later publication [4].…”

Bonding in Ground-State and Excited-State A+·Rg van der Waals Ions (A = Atom, Rg = Rare-Gas Atom): A Model-Potential Analysis

“…In this review, we discuss some of these data in the context of the "model-potential" analysis we have recently developed (building on earlier less complete or less general approaches 7,64 ) to treat A + /Rg "physical" bonding. 101,168,169 While there are obviously other ways to view the bonding, such as using molecular orbital or Lewis acid/base ideas (as discussed above), the physical model-potential approach is quantitative, general, and remarkably successful in rationalizing such bonding. We therefore leave to other authors any discussion of A + ‚Rg bonding within the context of such qualitative bonding models.…”

Long-range dispersion coefficients for Li, Li+, and Be+ interacting with the rare gases

“…[10][11][12][13] There have been some previous theoretical investigations of the Li-rare gas dispersion interactions, 14,15 with the most comprehensive work being the recent compilations by Zhang et al 16,17 Collisions between the alkaline-earth ions and the rare gases ͑RGs͒ occur in a number of different contexts. 21 Another application concerns the transport of alkaline-earth cations through rare gases. [18][19][20] The dispersion interaction can also play an important part of binding between metal ions and the rare gases.…”