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

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 (ΔE^orb) 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 ΔE^orb 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.