How Far Can One Push the Noble Gases Towards Bonding?: A Personal Account

Abstract

Noble gases (Ngs) are the least reactive elements in the periodic table towards chemical bond formation when compared with other elements because of their completely filled valence electronic configuration. Very often, extreme conditions like low temperatures, high pressures and very reactive reagents are required for them to form meaningful chemical bonds with other elements. In this personal account, we summarize our works to date on Ng complexes where we attempted to theoretically predict viable Ng complexes having strong bonding to synthesize them under close to ambient conditions. Our works cover three different types of Ng complexes, viz., non-insertion of NgXY type, insertion of XNgY type and Ng encapsulated cage complexes where X and Y can represent any atom or group of atoms. While the first category of Ng complexes can be thermochemically stable at a certain temperature depending on the strength of the Ng-X bond, the latter two categories are kinetically stable, and therefore, their viability and the corresponding conditions depend on the size of the activation barrier associated with the release of Ng atom(s). Our major focus was devoted to understand the bonding situation in these complexes by employing the available state-of-the-art theoretic tools like natural bond orbital, electron density, and energy decomposition analyses in combination with the natural orbital for chemical valence theory. Intriguingly, these three types of complexes represent three different types of bonding scenarios. In NgXY, the strength of the donor-acceptor Ng→XY interaction depends on the polarizing power of binding the X center to draw the rather rigid electron density of Ng towards itself, and sometimes involvement of such orbitals becomes large enough, particularly for heavier Ng elements, to consider them as covalent bonds. On the other hand, in most of the XNgY cases, Ng forms an electron-shared covalent bond with X while interacting electrostatically with Y representing itself as [XNg]+Y−. Nevertheless, in some of the rare cases like NCNgNSi, both the C-Ng and Ng-N bonds can be represented as electron-shared covalent bonds. On the other hand, a cage host is an excellent moiety to examine the limits that can be pushed to attain bonding between two Ng atoms (even for He) at high pressure. The confinement effect by a small cage-like B12N12 can even induce some covalent interaction within two He atoms in the He2@B12N12 complex.

Changes in Structure and Reactivity of Ng2 Encapsulated in Fullerenes: A Density Functional Theory Study

“…Previously, by heating fullerenes at 650°C under 3,000 atmosphere, helium (Saunders et al, 1993 ), neon (Saunders et al, 1993 ), argon (DiCamillo et al, 1996 ), krypton (Yamamoto et al, 1999 ), and xenon (Syamala et al, 2002 ) noble gas atoms were experimentally introduced into the fullerene cage. This experimental realization of fullerene encapsulation demonstrates the feasibility, and there were reports of experimental realization of noble gas dimers encapsulation in fulerenes (Saunders et al, 1996 ; Laskin et al, 1998 ; Peres et al, 2001 ; Popov et al, 2013 ; Saha et al, 2019 ). Also, earlier, Krapp and Frenking performed a computational study on Ng 2 @C 60 (Ng = He, Ne, Ar, Kr, and Xe) systems from the bonding perspective and concluded that He 2 @C 60 and Ne 2 @C 60 were weakly bonded van der Waals complexes (Krapp and Frenking, 2007 ).…”

Protonated and Cationic Helium Clusters

“…Strongly electronegative ligands, such as O and F atoms, have been found to be able to form neutral, covalently bound structures with Ar, Kr, and Xe, with the latter in particular displaying a rich chemical nature, but neutral compounds containing bound He and Ne have generally not been achieved [1][2][3]. Exotic exceptions to this are compounds formed under extreme pressures [4] or by trapping noble gases in molecular cages [3,5,6]. However, molecular ions containing covalently bound rare gases, including He and Ne, can be readily formed and have been studied since the discovery of the HeH + ion nearly a century ago [7].…”

Charge-Shift Bonding in Xenon Hydrides: An NBO/NRT Investigation on HXeY···HX (Y = Cl, Br, I; X = OH, Cl, Br, I, CCH, CN) via H-Xe Blue-Shift Phenomena

“…However, significant progress has been made in noble-gas chemistry during the past 20 years. Besides noble-gas hydrides (Khriachtchev et al, 2009), noble gas-noble metal complexes have been studied for their thermodynamic and kinetic stabilities in theory (Jana et al, 2017(Jana et al, , 2018aPan et al, 2019;Saha et al, 2019). Experimentally, about 30 noble-gas hydrides had been identified by the beginning of 1995 (Pettersson et al, 1995;Räsänen et al, 2000;Lundell et al, 2002;Feldman et al, 2003;Khriachtchev et al, 2003Khriachtchev et al, , 2009Duarte and Khriachtchev, 2017).…”