The chemistry of xenon(IV)

Noble Gas Anions: An Overview of Strategies and Bonding Motifs

This review article aims to provide an overview of the strategies employed to prepare noble gas anions under different environments and experimental conditions, and of the bonding motifs typically occurring in these species. Observed systems include anions fixed into synthesized salts, detected in the gas phase or in high-pressure devices. The major role of the theoretical calculations is also highlighted, not only in support of the experiments, but also as effective in predicting still unreported species. The chemistry of noble gas anions overall appears as a varied and rich paint, offering fascinating opportunities for both experimentalists and theoreticians.

Hole interactions of aerogen oxides with Lewis bases: an insight into σ-hole and lone-pair-hole interactions

σ-Hole and lone-pair (lp)-hole interactions of aerogen oxides with Lewis bases (LB) were comparatively inspected in terms of quantum mechanics calculations. The ZOn ⋯ LB complexes (where Z = Kr and Xe, n = 1, 2, 3 and 4, and LB = NH3 and NCH) showed favourable negative interaction energies. The complexation features were explained in light of σ-hole and lp-hole interactions within optimum distances lower than the sum of the respective van der Waals radii. The emerging findings outlined that σ-hole interaction energies generally enhanced according to the following order: KrO4 ⋯ < KrO⋯ < KrO3⋯ < KrO2⋯LB and XeO4⋯ < XeO⋯ < XeO2⋯ < XeO3⋯LB complexes with values ranging from -2.23 to -12.84 kcal mol-1. Lp-hole interactions with values up to -5.91 kcal mol-1 were shown. Symmetry-adapted perturbation theory findings revealed the significant contributions of electrostatic forces accounting for 50-65% of the total attractive forces within most of the ZOn⋯LB complexes. The obtained observations would be useful for the understanding of hole interactions, particularly for the aerogen oxides, with application in supramolecular chemistry and crystal engineering.

Xenon Derivatives as Aerogen Bond-Donating Catalysts for Organic Transformations: A Theoretical Study on the Metaphorical "Spherical Cow in a Vacuum" Provides Insights into Noncovalent Organocatalysis

Computations indicate that cationic and noncharged xenon derivatives should exhibit higher catalytic activity than their iodine-based noncovalent organocatalytic congeners. Perfluorophenyl xenonium(II) is expected to demonstrate the best balance between catalytic activity and chemical stability for use in organocatalysis. Comparing its catalytic activity with that of isoelectronic perfluoroiodobenzene indicates that the high catalytic activity of cationic noncovalent organocatalysts is predominantly attributed to the electrostatic interactions with the reaction substrates, which cause the polarization of ligated species during the reaction progress. In contrast, the electron transfer and covalent contributions to the bonding between the catalyst and substrate have negligible effects. The dominant effect of electrostatic interactions results in a strong negative correlation between the calculated Gibbs free energies of activation for the modeled reactions and the highest potentials of the σ-holes on the central atoms of the catalysts. No such correlation is observed for noncharged catalysts.

-Comparison of σ/ π-hole aerogen-bonding interactions based on C2H4···NgOX2 (Ng = Kr, Xe; X = F, Cl, Br) complexes

The geometric structure, energy properties, and electronic properties of the aerogen-bonding interaction formed by C₂H₄ and NgOX₂ (Ng = Kr, Xe; X = F, Cl, Br) have been studied at the B2PLYP-D3(BJ)/ aug-cc-pVTZ (PP) level. Two kinds of aerogen-bonding interactions were observed among the title systems: the σ-hole and the π-hole complexes. The σ-hole aerogen-bonding complex has a binding energy in the range of - 6.29 ~  - 8.17 kcal/mol, which is the most stable. The binding energies of C₂H₄···NgOX₂ increased as X = F < Cl < Br and Ng = KrOX₂ < XeOX₂ for the σ/π-hole aerogen-bonding complexes. The atoms in molecules (AIM), the non-covalent interaction (NCI) index, and the LMO-EDA energy decomposition analysis were adopted to study the nature of the σ/π-hole aerogen-bonding interaction. The results show that the electrostatic term contributes the most to the total interaction energy for the σ/π-hole aerogen-bonding complexes.

Noble Gas-Silicon Cations: Theoretical Insights into the Nature of the Bond

The structure, stability, and bonding situation of some exemplary noble gas-silicon cations were investigated at the MP2/aVTZ level of theory. The explored species include the mono-coordinated NgSiX3+ (Ng = He-Rn; X = H, F, Cl) and NgSiF22+ (Ng = He-Rn), the di-coordinated Ar2SiX3+ (X = H, F, Cl), and the “inserted” FNgSiF2+ (Ng = Kr, Xe, Rn). The bonding analysis was accomplished by the method that we recently proposed to assay the bonding situation of noblegas compounds. The Ng-Si bonds are generally tight and feature a partial contribution of covalency. In the NgSiX3+, the degree of the Ng-Si interaction mirrors the trends of two factors, namely the polarizability of Ng that increases when going from Ng = He to Ng = Rn, and the Lewis acidity of SiX3+ that decreases in the order SiF3+ > SiH3+ > SiCl3+. For the HeSiX3+, it was also possible to catch peculiar effects referable to the small size of He. When going from the NgSiF3+ to the NgSiF22+, the increased charge on Si promotes an appreciable increase inthe Ng-Si interaction, which becomes truly covalent for the heaviest Ng. The strength of the bond also increases when going from the NgSiF3+ to the “inserted” FNgSiF2+, likely due to the cooperative effect of the adjacent F atom. On the other hand, the ligation of a second Ar atom to ArSiX3+ (X = H, F, Cl), as to form Ar2(SiX3+), produces a weakening of the bond. Our obtained data were compared with previous findings already available in the literature.

Noble-gas compounds: A general procedure of bonding analysis

This paper accounts for a general procedure of bonding analysis that is, expectedly, adequate to describe any type of interaction involving the noble-gas (Ng) atoms. Building on our recently proposed classification of the Ng-X bonds (X = binding partner) [New J. Chem. 44, 15536 (2020)], these contacts are first distinguished into three types, namely, A, B, or C, based on the topology of the electron energy density H(r) and on the shape of its plotted form. Bonds of type B or C are, then, further assigned as B-loose (B^l) or B-tight (B^t) and C-loose (C^l) or C-tight (C^t) depending on the sign that H(r) takes along the Ng-X bond path located from the topological analysis of ρ(r), particularly at around the bond critical point (BCP). Any bond of type A, B^l/B^t, or C^l/C^t is, finally, assayed in terms of contribution of covalency. This is accomplished by studying the maximum, minimum, and average value of H(r) over the volume enclosed by the low-density reduced density gradient (RDG) isosurface associated with the bond (typically, the RDG isosurface including the BCP) and the average ρ(r) over the same volume. The bond assignment is also corroborated by calculating the values of quantitative indices specifically defined for the various types of interactions (A, B, or C). The generality of our taken approach should encourage its wide application to the study of Ng compounds.

A new krypton complex – Experimental and computational investigation of the krypton sulphur pentafluoride cation, [KrSF5]+, in the gas phase

The gas-phase reactions of noble gas (Ng) cations, namely Kr+ and Xe+, with SF6 were investigated experimentally by Fourier transform ion cyclotron resonance mass spectrometry and computationally using MP2 and...

Long-bonding and bonding nature in noble gas insertion compounds MNgBY of transition metal-boron bond

The nature of inert gas bonding has always been an important topic. The bonds of noble gases cover the entire range of chemical bonds, from the weakest van der Waals forces, to non-covalent interactions, and to covalent bonds. Two types of methods were used to investigate the properties of chemical bonds in the inert gas inserted compound MNgBY with the transition metal M = Cu/Ag/Au and substituents Y = O/S/NH, one based on orbital analysis and the other based on electron density analysis. The NBO/NRT analysis shows that in these compounds there exists long-bonding striding the noble gas between the transitional metal and boron, similar to the noble gas insertion compounds HNgX of hydrohalide, and so a three-center four-electron bond exists among the M-Ng-B part. The electron density analyses show that the M-Ng bond between the metal Cu/Ag/Au and noble gas and the Ng-B bond in the Cu/Ag compounds are partial covalent but the Ng-B bond in Au compounds is a typical covalent bond. The large relativistic effects of Au cause the bonds in Au compounds shorter and stronger than the bonds in Ag/Cu compounds. The properties of the M-Ng and Ng-B bonds are not affected by substituents Y, but the bond lengths are sensitive to substituents.

Syntheses and Characterizations of the Mixed Noble-Gas Compounds, [FKrII FXeII F][AsF6 ]⋅0.5 KrII F2 ⋅2 HF, ([KrII 2 F3 ][AsF6 ])2 ⋅XeIV F4 , and XeIV F4 ⋅KrII F2

Reaction of [XeF][AsF₆ ] with excess KrF₂ at -78 °C in anhydrous HF (aHF) solvent has yielded the first mixed Kr^II /Xe^II noble-gas compound, [FKrFXeF][AsF₆ ] ⋅0.5 KrF₂ ⋅2 HF, a salt of the [FKrFXeF]⁺ cation. The potent oxidative fluorinating properties of Kr^II fluoride species resulted in oxidation of Xe^II to Xe^IV in aHF at -60 °C to form the mixed Kr^II /Xe^IV cocrystals, ([Kr₂ F₃ ][AsF₆ ])₂ ⋅XeF₄ and XeF₄ ⋅KrF₂ . Further decomposition at 22 °C resulted in oxidation of Xe^IV to Xe^VI to give the recently reported Kr^II /Xe^VI complexes, [F₅ Xe(FKrF)_n ][AsF₆ ] (n=1, 2), [F₅ Xe][AsF₆ ], and a new Kr^II /Xe^VI complex, [(F₅ Xe)₂ (μ-FKrF)(AsF₆ )₂ ], which was characterized by low-temperature (LT) Raman spectroscopy. The [FKrFXeF][AsF₆ ]⋅0.5 KrF₂ ⋅2 HF, ([Kr₂ F₃ ][AsF₆ ])₂ ⋅XeF₄  , and XeF₄ ⋅KrF₂ compounds were characterized by LT Raman spectroscopy and single-crystal X-ray diffraction. Quantum-chemical calculations were used to assess the bonding in [FKrFXeF]⁺ , [Kr₂ F₃ ]⁺ , and [Xe₂ F₃ ]⁺ and to aid in their vibrational assignments.

New Perspectives in the Noble Gas Chemistry Opened by Electrophilic Anions

Binding of noble gases (NGs) is commonly considered to be the realm of highly reactive electophiles with cationic or at least non-charged character. Herein, we summarize our latest results evidencing that the incorporation of a strongly electrophilic site within a rigid cage-like anionic structure offers several advantages that facilitate the binding of noble gases and stabilize the formed NG adducts. The anionic superelectrophiles investigated by us are based on the closo-dodecaborate dianion scaffold. The record holder [B₁₂(CN)₁₁]⁻ binds spontaneously almost all members of the NG family, including the very inert argon at room temperature and neon at 50 K in the gas phase of mass spectrometers. In this perspective, we summarize the argumentation for the advantages of anionic electrophiles in binding of noble gases and explain them in detail using several examples. Then we discuss the next steps necessary to obtain a comprehensive understanding of the binding properties of electrophilic anions with NGs. Finally, we discuss the perspective to prepare bulk ionic materials containing NG derivatives of the anionic superelectophiles. In particular, we explore the role of counterions using computational methods and discuss the methodology, which may be used for the actual preparation of such salts.

Noble Gas Bonding Interactions Involving Xenon Oxides and Fluorides

Noble gas (or aerogen) bond (NgB) can be outlined as the attractive interaction between an electron-rich atom or group of atoms and any element of Group-18 acting as an electron acceptor. The IUPAC already recommended systematic nomenclature for the interactions of groups 17 and 16 (halogen and chalcogen bonds, respectively). Investigations dealing with noncovalent interactions involving main group elements (acting as Lewis acids) have rapidly grown in recent years. They are becoming acting players in essential fields such as crystal engineering, supramolecular chemistry, and catalysis. For obvious reasons, the works devoted to the study of noncovalent Ng-bonding interactions are significantly less abundant than halogen, chalcogen, pnictogen, and tetrel bonding. Nevertheless, in this short review, relevant theoretical and experimental investigations on noncovalent interactions involving Xenon are emphasized. Several theoretical works have described the physical nature of NgB and their interplay with other noncovalent interactions, which are discussed herein. Moreover, exploring the Cambridge Structural Database (CSD) and Inorganic Crystal Structure Database (ICSD), it is demonstrated that NgB interactions are crucial in governing the X-ray packing of xenon derivatives. Concretely, special attention is given to xenon fluorides and xenon oxides, since they exhibit a strong tendency to establish NgBs.

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.

Covalent and Non-covalent Noble Gas Bonding Interactions in XeF n Derivatives (n = 2-6): A Combined Theoretical and ICSD Analysis

A noble gas bond (also known in the literature as aerogen bond) can be defined as the attractive interaction between any element of group-18 acting as a Lewis acid and any electron rich atom of group of atoms, thus following the IUPAC recommendation available for similar π,σ-hole interactions involving elements of groups 17 (halogens) and 16 (chalcogens). A significant difference between noble gas bonding (NgB) and halogen (HaB) or chalcogen (ChB) bonding is that whilst the former is scarcely found in the literature, HaB and ChB are very common and their applications in important fields like catalysis, biochemistry or crystal engineering have exponentially grown in the last decade. This article combines theory and experiment to highlight the importance of non-covalent NgBs in the solid state of several xenon fluorides [XeF_n]^m+ were the central oxidation state of Xe varies from +2 to +6 and the number of fluorine atoms varies from n = 2 to 6. The compounds with an odd number of fluorine atoms (n = 3 and 5) are cationic (m = 1). The Inorganic Crystal Structural Database (ICSD) strongly evidences the relevance of NgBs in the solid state structures of xenon derivatives. The ability of Xe compounds to participate in π,σ-hole interactions has been studied using different types of electron donors (Lewis bases and anions) using DFT calculations (PBE1PBE-D3/def2-TZVP) and the molecular electrostatic potential (MEP) surfaces.

Xenon in oxide frameworks: at the crossroads between inorganic chemistry and planetary science

The chemistry of noble gases was for a long time dominated by fluoride-bearing compounds of xenon. However, the last two decades have brought new insights into the chemistry of xenon oxides and oxysalts, including insights involving a novel type of non-covalent interaction (aerogen bonding), discoveries of new xenon oxides, oxide perovskite frameworks and evidence for an abrupt increase of xenon reactivity under extreme pressure-temperature conditions. The complex implementation of these findings could facilitate the development of explanations for long-standing interdisciplinary problems, such as the depletion of heavy noble gases in contemporary planetary atmospheres - the cosmochemical enigma known as the "missing xenon" paradox.

First steps towards a stable neon compound: observation and bonding analysis of [B12(CN)11Ne]. Chem Commun (Camb) 2020;56:4591-4594. [PMID: 32207481 DOI: 10.1039/d0cc01423k]

Noble gas (Ng) containing molecular anions are much scarcer than Ng containing cations. No neon containing anion has been reported so far. Here, the experimental observation of the molecular anion [B₁₂(CN)₁₁Ne]^- and a theoretical analysis of the boron-neon bond is reported.

Heavy atom tunnelling on XeF6 pseudorotation

XeF₆ has multiple C_3v equivalent minima due to the Jahn–Teller effect. Through computational means we prove that the rearrangement between isomers occurs through fluorine quantum mechanical tunnelling.

Classifying the chemical bonds involving the noble-gas atoms

The Ng–X bonds are classified into covalent (Cov), and different types of non-covalent (nCov), or partially-covalent (pCov) interactions.

Prediction of the Reactivity of Argon with Xenon under High Pressures

Pressure significantly modifies the microscopic interactions in the condense phase, leading to new patterns of bonding and unconventional chemistry. Using unbiased structure searching techniques combined with first-principles calculations, we demonstrate the reaction of argon with xenon at a pressure as low as 1.1 GPa, producing a novel van der Waals compound XeAr₂. This compound is a wide-gap insulator and crystallizes in a MgCu₂-type Laves phase structure. The calculations of phonon spectra and formation enthalpy indicate that XeAr₂ would be stable without any phase transition or decomposition at least up to 500 GPa.

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

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.

The HKrCCH⋯CO2 complex: an ab initio and matrix-isolation study

Structure of the HKrCCH⋯CO₂ complex prepared in a low-temperature krypton matrix.

Aerogen bonds formed between AeOF2 (Ae = Kr, Xe) and diazines: comparisons between σ-hole and π-hole complexes

The interaction between KrOF₂ or XeOF₂ and the 1,2, 1,3, and 1,4 diazines is characterized chiefly by a Kr/XeN aerogen bond, as deduced from ab initio calculations. The most stable dimers take advantage of the σ-hole on the aerogen atom, wherein the two molecules lie in the same plane. The interaction is quite strong, as much as 18 kcal mol^-1. A second class of dimer geometry utilizes the π-hole above the aerogen atom in an approximate perpendicular arrangement of the two monomers; these structures are not as strongly bound: 6-8 kcal mol^-1. Both sorts of dimers contain auxiliary CHF H-bonds which contribute to their stability, but even with their removal, the aerogen bond energy remains as high as 14 kcal mol^-1. The nature and strength of each specific interaction is confirmed and quantified by AIM, NCI, NBO, and electron density shift patterns. There is not a great deal of sensitivity to the identity of either the aerogen atom or the position of the two N atoms in the diazine.

Photochemistry of the H2O/CO System Revisited: The HXeOH···CO Complex in a Xenon Matrix

We report on the complex of a noble-gas hydride HXeOH with carbon monoxide. This species is prepared via the annealing-induced H + Xe + OH···CO reaction in a xenon matrix, the OH···CO complexes being produced by VUV photolysis of the H₂O···CO complexes. The H-Xe stretching mode of the HXeOH···CO complex absorbs at 1590.3 cm^-1 and it is blue-shifted by 12.7 cm^-1 from the H-Xe stretching band of HXeOH monomer. The observed blue shift indicates the stabilization of the H-Xe bond upon complexation, which is characteristic of complexes of noble-gas hydrides. The HXeOH···CO species is the first complex of a noble-gas hydride with carbon monoxide and the second observed complex of HXeOH. On the basis of the MP2/aug-cc-pVTZ-PP calculations, the experimental complex is assigned to the structure, where the carbon atom of CO interacts with the oxygen atom of HXeOH.

Cooperativity between the hydrogen bonding and σ-hole interaction in linear NCX···(NCH)n=2–5 and O3Z···(NCH)n=2–5 complexes (X = Cl, Br; Z = Ar, Kr): a comparative study

Quantum chemical calculations are performed to investigate the cooperativity of hydrogen bonding with halogen or aerogen bonding interactions in linear NCX···(NCH)n=2–5 and O3Z···(NCH)n=2–5 clusters, where X = Cl, Br and Z = Ar, Kr. To understand the cooperativity mechanism in these systems, the corresponding binary NCX···NCH and O3Z···NCH complexes are also considered. The binding distances, interaction energies, and bonding properties of the NCX···(NCH)n=2–5 and O3Z···(NCH)n=2–5 clusters are analyzed in detail. It is found that the cooperative effects in the hydrogen bonding tend to strengthen X···N and Z···N interactions. For both NCX···(NCH)n and O3Z···(NCH)n clusters, a small bond shrinkage is observed from n = 4 to n = 5, which suggests that the cooperativity effects are almost saturated in the larger clusters (n > 5). As the size of the X or Z atom is increased, the magnitude of the cooperative energy in these systems is also increased, which is mainly ascribed to changes in electrostatic potentials and orbital interactions. Our results indicate that the cooperative effects lead to a substantial change in the 14N nuclear quadrupole coupling constants of the NCH molecule.

Noble gas bond and the behaviour of XeO3under pressure

The covalent Xe–O bond lengths in XeO₃are elongated upon increasing the pressure, which is similar to the change observed with hydrogen bonds under pressure. Moreover, XeO₃rearranges in a highly-ordered manner by O hopping at about 2 GPa, which is analogous to the proton hopping observed among hydrogen bonds.

The classification and representation of main group element compounds that feature three-center four-electron interactions

This article provides a means to classify and represent compounds that feature 3-center 4-electron (3c-4e) interactions in terms of the number of electrons that each atom contributes to the interaction. Specifically, Class I 3c-4e interactions are classified as those in which two atoms provide one electron each and the third atom provides a pair of electrons (i.e. LX₂), while Class II 3c-4e interactions are classified as those in which two atoms each provide a pair of electrons and the third atom contributes none (i.e. L₂Z). These classes can be subcategorized according to the nature of the central atom. Thus, Class I interactions can be categorized according to whether the central atom provides one (i.e.μ-X) or two (i.e.μ-L) electrons, while Class II interactions can be categorized according to whether the central atom provides none (i.e.μ-Z) or two (i.e.μ-L) electrons. The use of appropriate structure-bonding representations for these various interactions provides a means to determine the covalent bond classification of the element of interest.

π-Hole aerogen bonding interactions

In this manuscript we combine high level ab initio calculations (RI-MP2/aug-cc-pVTZ) and the analysis of several crystal structures to demonstrate the existence of π-hole aerogen bonding interactions in Xe(iv) compounds. The ability of XeF4 and Xe(OMe)4 to interact with electron rich molecules is rationalized using several computational tools, including molecular electrostatic potential surfaces, energetic and geometric features of the complexes and "atoms in molecules" (AIM) and Natural Bond Orbital (NBO) analyses. We have found support for the π-hole interaction involving the xenon atom from the solid state architecture of several X-ray structures retrieved from the crystal structural depot. Particularly, π-hole aerogen bonding interactions are quite common in the solid state of Xe(IV) compounds.

On the nature of interactions in the F2 OXe(…) NCCH3 complex: Is there the Xe(IV)N bond?

Nature of the bonding in isolated XeOF2 molecule and F2 OXe(…) NCCH3 complexes have been studied in the gas phase (0 K) using Quantum Chemical Topology methods. The wave functions have been approximated at the MP2 and DFT levels of calculations, using the APFD, B3LYP, M062X, and B2PLYP functionals with the GD3 dispersion correction. The nature of the formal XeO bond in the XeOF2 monomer depends on the basis set used (all-electron vs. the ecp-28 approximation for Xe). Within the all-electron basis set approach the bond is represented by two bonding attractors, Vi = 1,2 (Xe,O), with total population of about 1.06e and highly delocalized electron density in both bonding basins. No bonding basins are observed using the ecp-28 approximation. These results shows that the nature of xenon-oxygen is complicated and may be described with mesomeric equilibrium of the Lewis representations: Xe((+)) O((-)) and Xe((-)) O((+)) . For both the xenon-oxygen and xenon-fluorine interactions the charge-shift model can be applied. The F2 OXe(…) NCCH3 complex exists in two structures: "parallel," stabilized by non-covalent C(…) O and Xe(…) N interactions and "linear" stabilized by the Xe(…) N interaction. Topological analysis of ELF shows that the F2 OXe(…) NCCH3 molecule appears as a weakly bound intermolecular complex. Intermolecular interaction energy components have also been studied using Symmetry Adapted Perturbation Theory. © 2016 Wiley Periodicals, Inc.

Perovskites with the Framework-Forming Xenon

The Group 18 elements (noble gases) were the last ones in the periodic system to have not been encountered in perovskite structures. We herein report the synthesis of a new group of double perovskites KM(XeNaO6) (M = Ca, Sr, Ba) containing framework-forming xenon. The structures of the new compounds, like other double perovskites, are built up of the alternating sequence of corner-sharing (XeO6) and (NaO6) octahedra arranged in a three-dimensional rocksalt order. The fact that xenon can be incorporated into the perovskite structure provides new insights into the problem of Xe depletion in the atmosphere. Since octahedrally coordinated Xe(VIII) and Si(IV) exhibit close values of ionic radii (0.48 and 0.40 Å, respectively), one could assume that Xe(VIII) can be incorporated into hyperbaric frameworks such as MgSiO3 perovskite. The ability of Xe to form stable inorganic frameworks can further extend the rich and still enigmatic chemistry of this noble gas.

Aerogen Bonding Interaction: A New Supramolecular Force?

We report evidence of the favorable noncovalent interaction between a covalently bonded atom of Group 18 (known as noble gases or aerogens) and a negative site, for example, a lone pair of a Lewis base or an anion. It involves a region of positive electrostatic potential (σ-hole), therefore it is a totally new and unexplored σ-hole-based interaction, namely aerogen bonding. We demonstrate for the first time the existence of σ-hole regions in aerogen derivatives by means of high-level ab initio calculations. In addition, several crystal structures retrieved from the Cambridge Structural Database (CSD) give reliability to the calculations. Energetically, aerogen bonds are comparable to hydrogen bonds and other σ-hole-based interactions but less directional. They are expected to be important in xenon chemistry.