Noble-Gas Chemistry More than Half a Century after the First Report of the Noble-Gas Compound

Recent development in the synthesis and characterization of noble-gas compounds is reviewed, i.e., noble-gas chemistry reported in the last five years with emphasis on the publications issued after 2017. XeF2 is commercially available and has a wider practical application both in the laboratory use and in the industry. As a ligand it can coordinate to metal centers resulting in [M(XeF2)x]n+ salts. With strong Lewis acids, XeF2 acts as a fluoride ion donor forming [XeF]+ or [Xe2F3]+ salts. Latest examples are [Xe2F3][RuF6]·XeF2, [Xe2F3][RuF6] and [Xe2F3][IrF6]. Adducts NgF2·CrOF4 and NgF2·2CrOF4 (Ng = Xe, Kr) were synthesized and structurally characterized at low temperatures. The geometry of XeF6 was studied in solid argon and neon matrices. Xenon hexafluoride is a well-known fluoride ion donor forming various [XeF5]+ and [Xe2F11]+ salts. A large number of crystal structures of previously known or new [XeF5]+ and [Xe2F11]+ salts were reported, i.e., [Xe2F11][SbF6], [XeF5][SbF6], [XeF5][Sb2F11], [XeF5][BF4], [XeF5][TiF5], [XeF5]5[Ti10F45], [XeF5][Ti3F13], [XeF5]2[MnF6], [XeF5][MnF5], [XeF5]4[Mn8F36], [Xe2F11]2[SnF6], [Xe2F11]2[PbF6], [XeF5]4[Sn5F24], [XeF5][Xe2F11][CrVOF5]·2CrVIOF4, [XeF5]2[CrIVF6]·2CrVIOF4, [Xe2F11]2[CrIVF6], [XeF5]2[CrV2O2F8], [XeF5]2[CrV2O2F8]·2HF, [XeF5]2[CrV2O2F8]·2XeOF4, A[XeF5][SbF6]2 (A = Rb, Cs), Cs[XeF5][BixSb1-xF6]2 (x = ~0.37–0.39), NO2XeF5(SbF6)2, XeF5M(SbF6)3 (M = Ni, Mg, Zn, Co, Cu, Mn and Pd) and (XeF5)3[Hg(HF)]2(SbF6)7. Despite its extreme sensitivity, many new XeO3 adducts were synthesized, i.e., the 15-crown adduct of XeO3, adducts of XeO3 with triphenylphosphine oxide, dimethylsulfoxide and pyridine-N-oxide, and adducts between XeO3 and N-bases (pyridine and 4-dimethylaminopyridine). [Hg(KrF2)8][AsF6]2·2HF is a new example of a compound in which KrF2 serves as a ligand. Numerous new charged species of noble gases were reported (ArCH2+, ArOH+, [ArB3O4]+, [ArB3O5]+, [ArB4O6]+, [ArB5O7]+, [B12(CN)11Ne]−). Molecular ion HeH+ was finally detected in interstellar space. The discoveries of Na2He and ArNi at high pressure were reported. Bonding motifs in noble-gas compounds are briefly commented on in the last paragraph of this review.


Introduction
After almost 20 years, when the Christe's paper "A renaissance in noble gas chemistry" appeared [1], we can say that the renaissance still lasts, although to a lesser extent than in the past. Times are changing, and with that, the fields of research in science. However, new topics are also present in noble-gas (Ng) chemistry. Beside classical Ng chemistry, which includes the syntheses of larger quantities of noble gas compounds (bulk-phase compounds), nowadays there are many other kinds of studies connected with the identification and characterization of molecules of Ng compounds. They include the preparations of Ng compounds in cold matrices, syntheses in liquid and supercritical noble gases, formations of Ng compounds under high pressures, and syntheses of neutral and gas-ion compounds in the gas phase [2]. Lastly, we should not forget to mention a large number of theoretical papers predicting new Ng compounds, which still need to be experimentally confirmed.

Xenon
Chemistry of xenon represents the field with the largest number of synthesized Ng compounds. Xenon(II) fluoride is the only Ng compound that is commercially available and has a wider practical application both in the laboratory use and in the industry. Its main use is in its ability of fluorination of various organic compounds [9,10] and as an etching reagent for surfaces of various metals, oxides, nitrides, etc. [11][12][13][14][15][16]. It is also used for the preparation of fluorographenes [17][18][19]. Its applicability for low-temperature insertion of fluorine into oxides systems has been demonstrated with the purpose of modification of magnetic and electronic properties, in particular superconductivity [20]. The 18 F labeled XeF 2 is used in nuclear medicine. This includes imaging techniques such as positron emission tomography (PET) as an 18 F source [21]. Xenon is shown to bind to other molecules not only under matrix isolation (Lewis acid-base complex of 1,2-azaborine and Xe) [22] but also bulk-phases with metal-xenon bonds have been prepared and their crystal structures determined [23]. In a recent work from surface science, the Xe-Ru interactions with a significant amount of charge transfer were demonstrated at room temperature and low pressures [24]. Using new strategies taking advantage of confinement effects, these new two-dimensional structures allow the study of noble gas metal interactions down to the atomic scale, using the very sensitive toolkit of surface science methodology. The general ideas for using confinement effects for favoring interactions of noble gases with other elements at mild conditions is described in a recent topical review [25].

Xenon(II)
Since the first reported case of a compound with a XeF 2 ligand coordinated to a metal center, i.e., [Ag(XeF 2 ) 2 ](AsF 6 ) [26], a large number of other examples have been synthesized [27]. A variety of compounds exist, where different numbers of XeF 2 molecules are bound to various metal centers yielding [M(XeF 2 ) x ] n+ cationic part. The oxidation states of the metals are M(I), M(II), or M(III). The most important aspects that influence the formation of XeF 2 coordination compounds are charge and the size of the cation, the type of the anion that compensates the positive charge of the cationic part, solubility of the salt, and the concentration of the XeF 2 ligand [27].

Xenon(VI) Fluoride and its [XeF5] + and [Xe2F11] + Salts
XeF6 was prepared and isolated in solid argon and neon matrices [30]. The IR spectra of 129 XeF6 and 136 XeF6 isotopologues, recorded in the neon matrix, agree with theoretical ones for the C3v conformer of the XeF6 molecule in the neon matrix. As an internal reference matrix-site and Xeisotope splitting of corresponding Xe-F and Xe=O stretching modes of XeOF4 were analyzed in solid neon [30].

Xenon(VI) Fluoride and its [XeF5] + and [Xe2F11] + Salts
XeF6 was prepared and isolated in solid argon and neon matrices [30]. The IR spectra of 129 XeF6 and 136 XeF6 isotopologues, recorded in the neon matrix, agree with theoretical ones for the C3v conformer of the XeF6 molecule in the neon matrix. As an internal reference matrix-site and Xeisotope splitting of corresponding Xe-F and Xe=O stretching modes of XeOF4 were analyzed in solid neon [30].  [33], and

Xenon(VI) Fluoride and its [XeF 5 ] + and [Xe 2 F 11 ] + Salts
XeF 6 was prepared and isolated in solid argon and neon matrices [30]. The IR spectra of 129 XeF 6 and 136 XeF 6 isotopologues, recorded in the neon matrix, agree with theoretical ones for the C 3v conformer of the XeF 6 molecule in the neon matrix. As an internal reference matrix-site and Xe-isotope splitting of corresponding Xe-F and Xe=O stretching modes of XeOF 4 were analyzed in solid neon [30].  +   Reactions between various amounts of XeF 2 and MF 2 , MF 3 , or MF 4 (M = Ti, Mn, Sn, Pb) with UV-photolyzed F 2 in liquid anhydrous hydrogen fluoride (aHF) [34] Figure 4).    5-anion built from ten TiF6 octahedra, sharing vertices in the shape of a double-star ( Figure  5). It crystallizes in two crystal modification at low (α-phase, 150 K) and ambient (β-phase, 296 K) temperatures.   Figure  6).  The crystal structure of [XeF5]5[Ti10F45] consists of [XeF5] + cations and the largest known discrete [Ti10F45] 5-anion built from ten TiF6 octahedra, sharing vertices in the shape of a double-star ( Figure  5). It crystallizes in two crystal modification at low (α-phase, 150 K) and ambient (β-phase, 296 K) temperatures.   Figure  6).    The attempts to grow single crystals in XeF6/MF4 (M = Sn, Pb) system were successful in three cases.  Figure  8).    Figure  8).

Xenon(VI) Oxide Compounds
One of the main obstacles in the research of XeO3 is its extreme sensitivity. Solid XeO3 detonates when subjected to mild thermal or mechanical shock. For these reasons, experiments are usually limited to small quantities of XeO3 and its compounds (up to 20 mg). Although crystal structures of β-XeF 5 Mn(SbF 6 ) 3 and αand β-modifications of XeF 5 Pd(SbF 6 ) 3 , with larger M 2+ cations, differ from the other XeF 5 M(SbF 6 ) 3 compounds the main structure motif is preserved, i.e., a three-dimensional (3D) framework constructed of the M 2+ cations and SbF 6 units and [XeF 5 ] + cations located inside the cavities (Figure 14).

Xenon(VI) Oxide Compounds
One of the main obstacles in the research of XeO 3 is its extreme sensitivity. Solid XeO 3 detonates when subjected to mild thermal or mechanical shock. For these reasons, experiments are usually limited to small quantities of XeO 3 and its compounds (up to 20 mg).

Krypton
Beside xenon, krypton is the only noble gas where isolable compounds in macroscopic amounts can be prepared [44]. Its chemistry is limited to the +2 oxidation state. Known krypton(II) compounds are less stable than corresponding xenon(II) salts. Compounds in which KrF2 serves as a ligand towards Lewis centers have been only recently prepared [45]. The latest example is the synthesis and

Krypton
Beside xenon, krypton is the only noble gas where isolable compounds in macroscopic amounts can be prepared [44]. Its chemistry is limited to the +2 oxidation state. Known krypton(II) compounds are less stable than corresponding xenon(II) salts. Compounds in which KrF 2 serves as a ligand towards Lewis centers have been only recently prepared [45]. The latest example is the synthesis and crystal structure determination of [Hg(KrF 2 ) 8 ][AsF 6 ] 2 ·2HF [45]. It is the first homoleptic KrF 2 coordination complex of a metal cation ( Figure 19).  [45]. It is the first homoleptic KrF2 coordination complex of a metal cation ( Figure 19).

Chemistry of Argon
Experimental chemistry of argon is limited to studies in the low-temperature matrices and molecules observed in the gas phase. The HArF is presently the only experimental known neutral molecule containing a chemically bound argon atom that is stable in a low temperature argon matrix

Chemistry of Argon
Experimental chemistry of argon is limited to studies in the low-temperature matrices and molecules observed in the gas phase. The HArF is presently the only experimental known neutral molecule containing a chemically bound argon atom that is stable in a low temperature argon matrix [52]. Another story is the number of reported charged species detected in the gas phase. Some recent examples include observation of ArCH 2 + in mass spectrometry experiments [53] and the production of ArOH + molecular ion in a pulsed discharge/supersonic expansion of argon seeded with water vapor [54].

Chemistry of Neon and Helium
Astrophysical detection of HeH + in nearby interstellar space [57] is one of the greatest discoveries in molecular astrophysics. It was the first molecule to form after the big bang [58]. The synthesis of the isolable compounds containing neon and helium still remains an open challenge [8]. In 1992 it was reported that high pressure stabilizes the formation of a solid van der Waals compound of composition He(N 2 ) 11 , obtained by compression of helium-nitrogen mixtures [59]. A few years ago, the discovery of non-inclusion compound Na 2 He at pressures higher than 113 GPa was reported [60]. It was described as an electride, i.e., a crystal made of positively charged ionic cores and with strongly localized valence electrons playing the role of anions. Peculiar Na 2 He calls for making our definitions of "compound" more precise [8]. Defect perovskites (He 2-x x )(CaZr)F 6 can be prepared by inserting helium into CaZrF 6 at high pressure [61]. Despite these, helium and also neon have not been forced to form genuine chemical compounds in neutral entities to this day [8]. One of the first steps towards a stable neon compound is claimed to be the experimental observation of the molecular anion [B 12 (CN) 11 Ne] − [62].

Bonding Motifs in Noble-Gas Compounds
Despite the inertness of noble gases, Ng chemistry is rich in a variety of species with different bonding motifs [2,4,63]. The bonding motifs are a consequence of the polarization of the spherical electronic cloud of the Ng atoms by binding partners and they range from very weak 'dispersion' to stronger 'induced-dipole' interactions resulting in neutral and ionic 'complexes' of the noble gases, whose character ranges from fragile van der Waals adducts to definitely stable compounds [2].
Clusters of Ng atoms are held together by dispersion forces [2]. In various monocoordinated Ng species [X(Ng)n (n ≥ 1)], the character of the occurring interactions may span from weakly bound van der Waals adducts, held together by dispersion forces, to strongly bound covalent species [2]. Dicoordinated ('inserted') compounds have a general formula XNgY (X different or the same as Y), the Ng atom being involved in definitely recognizable interactions with both X and Y [2]. They can be neutral or ionic. The bonding of the noble-gas hydrides with the common formula HNgY compounds can be described as (HNg) + Y − where (HNg) + is mainly covalently bonded and the interaction of the (HNg) + and Y − is strongly ionic [63]. The most frequently drawn picture of the chemical bonding in XeF 2 is the molecular orbital approach involving three-center, four-electron bonds 3-center 4-electron bonding (3c-4e) [28,64,65]. A single bond is thus spread over the F-Xe-F system. XeF 2 is a fluoride ion donor and it can form everything from weakly bond complexes to XeF + salts with ionic formulation. In the weakly bond complexes [66], the XeF 2 geometry stays intact. In XeF 2 adducts as XeF 2 ·CrOF 4 [28] there is a slight elongation of Xe-F b (F b = bridging F atom interacting with Cr) bond, while the Xe-F t (F t = terminal F atom) bond is slightly shortened [2]. The elongation (weakening) of Xe-F b and shortening (strengthening) of Xe-F t bond is observed also in [M(XeF 2 ) x ] n+ [A] n− compounds where XeF 2 is coordinated only by one of its fluorine atoms [27,67]. The distortion of XeF 2 is visible in its Raman spectrum. Its vibrational band at 497 cm −1 is replaced by two vibrational bands. One is higher and one lower than 497 cm −1 [67]. Numerous products of reactions between XeF 2 and various fluorides are formulated as [XeF] + salts [28]. Although their formulations imply a simple ionic nature, there is no complete F − transfer between XeF 2 and fluoride ion acceptor [28]. The [XeF] + cation and its anion interact through Xe-F···M bridges and the difference between Xe-F b and Xe-F t bond lengths is much more pronounced. Structural and vibrational evidences for the ionization pathway XeF 2 → XeF + + F − were obtained studying the XeF 2 /XeF 5 AsF 6 system [68] where at least five distinct well-characterized phases exist in the phase diagram of the XeF 2 /XeF 5 AsF 6 mixture, and each of them exhibit a more or less pronounced dissociation of XeF 2 into the ionic [XeF + ]···[A-F − ] form (A = Lewis acid) [63].
The bonding in XeF 6 has caused considerable controversy that is not completely resolved [69][70][71]. The XeF 6 is the strongest fluoride ion donor among XeF 2 , XeF 4 , and XeF 6 . It forms a large number of [XeF 5 ] + and [Xe 2 F 11 ] + salts with a variety of Lewis acid fluorides and oxyfluorides [4,5]. The [XeF 5 ] + cation geometry may be described in terms of pseudo-octahedral AX 5 E VSEPR arrangements of five bond pairs (X) and the lone electron pair (E) around Xe (A) which give rise to a square-pyramidal geometry of Xe and five F atoms [38,72]. Although these compounds are ionic in their nature, the [XeF 5 ] + and [Xe 2 F 11 ] + cations are extensively associated with their anions through Xe···F (F belongs to the anion) secondary bonding interactions [73].
In XeO 3 adducts, the xenon-ligand bonds may be described as predominantly electrostatic, (weakly covalent) interactions between the highly electrophilic σ-holes of the xenon atom and the electronegative ligand atom [42].

Conclusions
This review shows that noble-gas chemistry is still interesting not just to chemists (at least to some of us) but also to astronomers, geologists, etc. For decades, astronomers have pursued for helium hydride HeH + , made of the two most common elements in the universe. In the laboratory the ion was discovered in 1925 [74]; however, we had to wait almost 100 years for the confirmation of the existence of HeH + in nearby interstellar space [57]. High-pressure and high-temperature (by laser-heating) study of the possible formation of stable compounds between Ar and Ni at thermodynamic conditions representative of the Earth's core resulted in the formation of ArNi compound (at 140 GPa and 1500 K) [75]. This result implies that the presence of argon in the Earth's core is highly probable [75].
Recent review about the noble-gas/noble-metal chemistry suggests an inflation of such complexes, not only in theory or in microscopic amount in cryogenic situation, but also in large-scale syntheses [76].
Additional stimulation in the noble-gas research represents the discovery of noble gas (or aerogen) bonding [77]. It is a novel type of weak attractive noncovalent interaction [78]. According to IUPAC is defined as the attractive interaction between an electron rich atom or group of atoms and any element of Group-18 acting as electron acceptor [79,80].

Funding:
The author acknowledges the financial support from the Slovenian Research Agency (research core funding No. P1-0045; Inorganic Chemistry and Technology).