NgMCp+: Noble Gas Bound Half-Sandwich Complexes (Ng = He–Rn, M = Be–Ba, and Cp = η5-C5H5)

Abstract

Structures, bonding, and stability of half-sandwich complexes with general formula, NgMCp+ (Ng = He–Rn, M = Be–Ba, Cp = η5-C5H5) are analyzed through ab initio computation. MCp+ complexes possess remarkable Ng binding ability, particularly for M = Be and Mg. While for Ar–Rn bound analogues the bond dissociation energy in the former complex ranges within 17.5–28.0 kcal mol–1, it becomes 10.4–18.7 kcal mol–1 in the latter complex. In fact, BeCp+ is able to form a strong bond with the two most inert elements, He and Ne. Although the Ng binding ability of MCp+ gradually diminishes in moving from Be to Ba, the corresponding free energy change values show that Kr–Rn bound complexes involving the heavier congeners of Mg would remain in the bound state avoiding dissociation into Ng and MCp+. The nature of the Ng–M bond is characterized by natural bond orbital, electron density and energy decomposition analyses in conjunction with the natural orbital for chemical valence (EDA-NOCV) analysis. While the electron density analysis reveals that Ng–Be (Ng = Kr, Xe, Rn) and Ng–Mg (Ng = Xe, Rn) bonds are partly covalent in nature, the orbital interaction (ΔEorb) is found to be the most important term in the Ng–M attractive energy as revealed by the EDA-NOCV. For all Ngs, the major contribution toward the ΔEorb energy term originates from Ng→MCp+ σ-donation. Additionally, CpBeNgF (Ng = Xe, Rn) and CpNgF (Ng = Kr–Rn) are found to be viable systems with kinetic protection for the exergonic dissociation channels, CpBeNgF → Ng + CpBeF and CpNgF → Ng + CpF, respectively, where the activation free energy barrier in the latter systems (24.1–34.7 kcal mol–1) is significantly larger than that in the former ones (6.6–8.9 kcal mol–1). CpNgF (Ng = Kr–Rn) complexes are predicted to be stable even above 300 K, whereas CpBeNgF (Ng = Xe, Rn) would be viable up to ∼100 K. While the F-Ng bonds are ionic in nature, the Ng–Be and Ng–C bonds in these complexes have significant covalent character.