Structure and stability of noble gas bound EX3+ compounds (E = C, Ge, Sn, Pb; X = H, F, Cl, Br)

Exploring the Nature of Chemical Bonding between Noble Gases and Hypercoordinate Group 13 Compounds: Beyond Boron

In this article, we systematically study the stability and chemical bond nature of EH4Ng+ compounds (E = Al-Tl; Ng = He-Rn) at the CCSD(T) and ωB97XD levels of theory. Thermochemical calculations obtained by exploring different dissociation pathways show that these compounds could be stable at low temperatures. In addition, studied compounds have a strong E-Ng bond, which has been characterized using different methodologies such as quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO) theory, and natural energy decomposition analysis (NEDA). Results indicate that the nature of the chemical bond is predominantly covalent, especially in the case those including the heavier gases (Ar-Rn), occurring through a charge transfer from the noble gas to the group 13 element. However, the electrostatic contribution is also important in the stabilization of this bond. This study extends the universe of group 13 molecules containing noble gas bonds beyond boron and other elements from the second period.

Structure, stability, reactivity and bonding in noble gas compounds

Noble gases (Ngs) are recognized as the least reactive elements due to their fully filled valence electronic configuration. Their reluctance to engage in chemical bond formation necessitates extreme conditions such as low temperatures, high pressures, and reagents with high reactivity. In this Perspective, we discuss our endeavours in the theoretical prediction of viable Ng complexes, emphasizing the pursuit of synthesizing them under nearly ambient conditions. Our research encompasses various bonding categories of Ng complexes and our primary aim is to comprehend the bonding mechanisms within these complexes, utilizing state-of-the-art theoretical tools such as natural bond orbital, energy decomposition, and electron density analyses. These complex types manifest distinct bonding scenarios. In the non-insertion type, the donor-acceptor interaction strength hinges on the polarizing ability of the binding atom, drawing the electron density of the Ng towards itself. In certain instances, especially with heavier Ng elements, this interaction reaches a magnitude where it can be considered a covalent bond. Conversely, in most insertion cases, the Ng prefers to share electrons to form a covalent bond on one side while interacting electrostatically on the other side. In rare cases, both bonds may be portrayed as electron-shared covalent bonds. Furthermore, a host cage serves as an excellent platform to explore the limits of achieving Ng-Ng bonds (even for helium), under high pressure.

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.

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.

Dative versus electron-sharing bonding in the isoelectronic argon compounds ArR+ (R = CH3, NH2, OH, and F)

For the series of isoelectronic ArR⁺ (R = CH₃, NH₂, OH, and F) complexes, the nature of the bonding between Ar and R shifts from an Ar → R⁺ dative σ bond in ArCH₃⁺ and ArNH₂⁺ to an Ar⁺–R electron-sharing σ bond in ArOH⁺ and ArF⁺.

Noble Gas Binding Ability of an Au(I) Cation Stabilized by a Frustrated Lewis Pair: A DFT Study

The noble gas (Ng) binding ability of a monocationic [(FLP)Au]⁺ species has been investigated by a computational study. Here, the monocationic [(FLP)Au]⁺ species is formed by coordination of Au(I) cation with the phosphorous (Lewis base) and the boron (Lewis acid) centers of a frustrated Lewis pair (FLP). The bonds involving Au and P, and Au and B atoms in [(FLP)Au]⁺ are partially covalent in nature as revealed by Wiberg bond index (WBI) values, electron density analysis and energy decomposition analysis (EDA). The zero point energy corrected bond dissociation energy (D₀), enthalpy and free energy changes are computed for the dissociation of Au-Ng bonds to assess the Ng binding ability of [(FLP)Au]⁺ species. The D₀ ranges from 6.0 to 13.3 kcal/mol, which increases from Ar to Rn. Moreover, the dissociation of Au-Ng bonds is endothermic as well as endergonic for Ng = Kr-Rn, whereas the same for Ng = Ar is endothermic but exergonic at room temperature. The partial covalent character of the bonds between Au and Ng atoms is demonstrated by their WBI values and electron density analysis. The Ng atoms get slight positive charges of 0.11–0.23 |e|, which indicates some amount of charge transfer takes place from it. EDA demonstrates that electrostatic and orbital interactions have equal contributions to stabilize the Ng-Au bonds in the [(FLP)AuNg]⁺ complex.

Unprecedented stability enhancement of multiply charged anions through decoration with negative electron affinity noble gases

The present communication reports unprecedented stabilization of multiply charged anion, B12F122-, through insertion of noble gas (Ng) atoms possessing negative electron affinity into B-F bonds, resulting in the formation of stable icosahedral B12Ng12F122-, where the HOMO is stabilized significantly and the binding energy of the second excess electron is increased remarkably. Unprecedented stability enhancement with Ng is attributed to a strong covalent B-Ng bond, increased charge delocalization and increased electrostatic interaction between the oppositely charged centers.

Intriguing structural, bonding and reactivity features in some beryllium containing complexes

We highlighted our contributions to Be chemistry which include bond-stretch isomerism in Be₃²⁻ species, Be complexes bound with noble gas, CO, and N₂, Be based nanorotors, and intriguing bonding situations in some Be complexes.

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.

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.

Systematic study of the substitution effect on the tetrel bond between 1,4-diazabicyclo[2.2.2]octane and TH3X

A tetrel bond was characterized in the complexes of 1,4-diazabicyclo[2.2.2]octane (DABCO) with TH3X (T = C, Si, Ge; X= -Me, -H, -OH, -NH2, -F, -Cl, -Br, -I, -CN, -NO2). DABCO engages in a weak tetrel bond with CH3X but a stronger one with SiH3X and GeH3X. SiH3X is favorable to bind with DABCO relative to GeH3X, inconsistent with the magnitude of the σ-hole on the tetrel atom. The methyl group in the tetrel donor weakens the tetrel bond but an enhancing effect is found for the other substituents, particularly -NO2. The substitution effect is also related to the nature of the tetrel atom. The halogen substitution from F to I has a weakening effect in the CH3X complex but an enhancing effect in the SiH3X complex and a negligible effect in the GeH3X complex. The above abnormal results found in these complexes can be partly attributed to the charge transfer from the lone pair on the nitrogen atom of DABCO into the anti-bonding orbital σ*(T-X) of TH3X. The stability of both SiH3X and GeH3X complexes is primarily controlled by electrostatic interactions and polarization.

Noble-Noble Strong Union: Gold at Its Best to Make a Bond with a Noble Gas Atom

This Review presents the current status of the noble gas (Ng)‐noble metal chemistry, which began in 1977 with the detection of AuNe⁺ through mass spectroscopy and then grew from 2000 onwards; currently, the field is in a somewhat matured state. On one side, modern quantum chemistry is very effective in providing important insights into the structure, stability, and barrier for the decomposition of Ng compounds and, as a result, a plethora of viable Ng compounds have been predicted. On the other hand. experimental achievement also goes beyond microscopic detection and characterization through spectroscopic techniques and crystal structures at ambient temperature; for example, (AuXe₄)²⁺(Sb₂F₁₁⁻)₂ have also been obtained. The bonding between two noble elements of the periodic table can even reach the covalent limit. The relativistic effect makes gold a very special candidate to form a strong bond with Ng in comparison to copper and silver. Insertion compounds, which are metastable in nature, depending on their kinetic stability, display an even more fascinating bonding situation. The degree of covalency in Ng–M (M=noble metal) bonds of insertion compounds is far larger than that in non‐insertion compounds. In fact, in MNgCN (M=Cu, Ag, Au) molecules, the M−Ng and Ng−C bonds might be represented as classical 2c–2e σ bonds. Therefore, noble metals, particularly gold, provide the opportunity for experimental chemists to obtain sufficiently stable complexes with Ng at room temperature in order to characterize them by using experimental techniques and, with the intriguing bonding situation, to explore them with various computational tools from a theoretical perspective. This field is relatively young and, in the coming years, a lot of advancement is expected experimentally as well as theoretically.

A theoretical investigation on boron–ligand cooperation to activate molecular hydrogen by a frustrated Lewis pair and subsequent reduction of carbon dioxide

Activation of molecular hydrogen by a B/N frustrated Lewis pair.

Cyanide-isocyanide isomerization: stability and bonding in noble gas inserted metal cyanides (metal = Cu, Ag, Au)

The internal isomerization, MNC ↔ MCN (M = Cu, Ag, Au), is investigated through quantum chemical computations. CuNC and AgNC are shown to be neither thermochemically nor kinetically stable against transformation to MCN. The free energy barrier (ΔG‡) for AuNC is somewhat considerable (7.1 kcal mol-1), indicating its viability, particularly at low temperature. Further, the Ng inserted analogues, MNgCN (M = Cu, Ag, Au; Ng = Xe, Rn) turn out to be thermochemically stable with respect to all possible dissociation channels but for two two-body dissociation channels, viz., MNgCN → Ng + MCN and MNgCN → Ng + MNC, which are connected to the internal isomerization processes, MNgCN → NgMCN and MNgCN → NgMNC, respectively. However, they are kinetically protected by substantial ΔG‡ values (11.8-15.4 kcal mol-1 for Cu, 9.8-13.6 kcal mol-1 for Ag, and 19.7-24.7 kcal mol-1 for Au). The pathways for such internal conversion are explored in detail. A thorough inspection of the bonding situation of the studied molecules, employing natural bond order, electron density, adaptive natural density partitioning, and energy decomposition analyses indicates that the M-Ng bonds in MNgCN and Ng-C bonds in AuNgCN can be represented as an electron-shared covalent bond. For the other Ng-C bonds, although an ionic description is better suited, the degree of covalent character is also substantial therein.

Boron Nanowheels with Axles Containing Noble Gas Atoms: Viable Noble Gas Bound M©B10- Clusters (M=Nb, Ta)

The viability of noble gas axled boron nanowheels Ng_n M©B₁₀^- (Ng=Ar-Rn; M=Nb, Ta; n=1, 2) is explored by ab initio computations. In the resulting Ng₂ -M complexes, the Ng-M-Ng nanorod passes through the center of the B₁₀^- ring, providing them with an inverse sandwich-like structure. While in the singly Ng bound analogue, the Ng binding enthalpy H_b at 298 K ranges from 2.5 to 10.6 kcal mol^-1 , in doubly Ng bound cases it becomes very low for the Ng₂ M©B₁₀^- →Ng+NgM©B₁₀^- dissociation channel, except for the case of Rn, for which the corresponding H_b values are 3.4 (Nb) and 4.0 kcal mol^-1 (Ta). For a given Ng, Ta has slightly higher Ng-binding ability than Nb. The corresponding free-energy changes indicate that these systems, particularly the Xe and Rn complexes, are good candidates for experimental realization in a low-temperature matrix. The Ng-M bonds were found to be covalent in nature, as reflected in their large Wiberg bond indices, formation of a 2c-2e σ orbital between Ng and M centers in natural bond orbital and adaptive natural density partitioning (AdNDP) analyses, and the short Ng-M distances. Energy decomposition analysis and a study on the natural orbitals for chemical valence show that the Ng-M contact is supported mainly by the orbital and electrostatic interactions, with almost equal contributions. Although both the Ng→M σ donation and Ng←M π backdonation play roles in the origin of orbital interaction, the former is significantly dominant over the latter. Further, AdNDP analysis indicates that the doubly aromatic character (both σ and π) in MB₁₀^- clusters is not perturbed by the interaction with Ng atoms.

Stable NCNgNSi (Ng=Kr, Xe, Rn) Compounds with Covalently Bound C-Ng-N Unit: Possible Isomerization of NCNSi through the Release of the Noble Gas Atom

Although the noble gas (Ng) compounds with either Ng-C or Ng-N bonds have been reported in the literature, compounds containing both bonds are not known. The first set of systems having a C-Ng-N bonding unit is predicted herein through the analysis of stability and bonding in the NCNgNSi (Ng=Kr-Rn) family. While the Xe and Rn inserted analogues are thermochemically stable with respect to all dissociation channels, but for the one producing CNSiN and free Ng, NCKrNSi has another additional three-body dissociation channel, NCKrNSi→CN+Kr+NSi, which is exergonic by -9.8 kcal mol^-1 at 298 K. This latter dissociation can be hindered by lowering the temperature. Moreover, the NCNgNSi→Ng+CNSiN dissociation is also kinetically prohibited by a quite high free energy barrier ranging from 25.2 to 39.3 kcal mol^-1 , with a gradual increase in going from Kr to Rn. Therefore, these compounds are appropriate candidates for experimental realization. A detailed bonding analysis by employing natural bond orbital, electron density, energy decomposition, and adaptive natural density partitioning analyses indicates that both Ng-N and C-Ng bonds in the title compounds are covalent in nature. In fact, the latter analysis indicates the presence of delocalized 3c-3e σ-bond within the C-Ng-N moiety and a totally delocalized 5c-2e σ-bond in these compounds. This is an unprecedented bonding characteristic in the sense that the bonding pattern in Ng inserted compounds is generally represented as the presence of covalent bond in one side of Ng, and the ionic interaction in the other side. Further, the dissociation of Ng from NCNgNSi facilitates the formation of a higher energy isomer of NCNSi, CNSiN, which cannot be formed from bare NCNSi as such, because of the very high free energy barrier associated with the isomeric transformation. Therefore, in the presence of Ng atoms it might be possible to detect the high energy isomer.

σ-Aromatic cyclic M3+ (M = Cu, Ag, Au) clusters and their complexation with dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gases and carbon monoxide

The structure, stability, bonding and σ-aromaticity in dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gas and carbon monoxide bound M₃⁺ (M = Cu, Ag, Au) complexes are analyzed.

Noble gas bound beryllium chromate and beryllium hydrogen phosphate: a comparison with noble gas bound beryllium oxide

Two new beryllium based compounds, beryllium hydrogen phosphate and beryllium chromate are found to have remarkable noble gas binding ability, particularly for Ar–Rn atoms.

Noble gas supported B3+ cluster: formation of strong covalent noble gas–boron bonds

Ar to Rn atoms formed exceptionally strong bonds with B₃⁺, where the Ng (HOMO) → B₃Ng₂⁺ (LUMO) σ-donation is the key term to stabilize the complexes.