Nature of the XeVI −N Bonds in F6 XeNCCH3 and F6 Xe(NCCH3 )2 and the Stereochemical Activity of Their Xenon Valence Electron Lone Pairs

Prediction of stable radon fluoride molecules and geometry optimization using first-principles calculations

Noble gases possess extremely low reactivity because their valence shells are closed. However, previous studies have suggested that these gases can form molecules when they combine with other elements with high electron affinity, such as fluorine. Radon is a naturally occurring radioactive noble gas, and the formation of radon-fluorine molecules is of significant interest owing to its potential application in future technologies that address environmental radioactivity. Nevertheless, because all isotopes of radon are radioactive and the longest radon half-life is only 3.82 days, experiments on radon chemistry have been limited. Here, we study the formation of radon molecules using first-principles calculations; additionally, possible compositions of radon fluorides are predicted using a crystal structure prediction approach. Similar to xenon fluorides, di-, tetra-, and hexafluorides are found to be stabilized. Coupled-cluster calculations reveal that RnF₆ stabilizes with O_h point symmetry, unlike XeF₆ with C_3v symmetry. Moreover, we provide the vibrational spectra of our predicted radon fluorides as a reference. The molecular stability of radon di-, tetra-, and hexafluoride obtained through calculations may lead to advances in radon chemistry.

Mixed Noble-Gas Compounds of Krypton(II) and Xenon(VI); [F5 Xe(FKrF)AsF6 ] and [F5 Xe(FKrF)2 AsF6 ]

The coordination chemistry of KrF₂ has been limited in contrast with that of XeF₂ , which exhibits a far richer coordination chemistry with main-group and transition-metal cations. In the present work, reactions of [XeF₅ ][AsF₆ ] with KrF₂ in anhydrous HF solvent afforded [F₅ Xe(FKrF)AsF₆ ] and [F₅ Xe(FKrF)₂ AsF₆ ], the first mixed krypton/xenon compounds. X-ray crystal structures and Raman spectra show the KrF₂ ligands and [AsF₆ ]^- anions are F-coordinated to the xenon atoms of the [XeF₅ ]⁺ cations. Quantum-chemical calculations are consistent with essentially noncovalent ligand-xenon bonds that may be described in terms of σ-hole bonding. These complexes significantly extend the XeF₂ -KrF₂ analogy and the limited chemistry of krypton by introducing a new class of coordination compound in which KrF₂ functions as a ligand that coordinates to xenon(VI). The HF solvates, [F₅ Xe(FH)AsF₆ ] and [F₅ Xe(FH)SbF₆ ], are also characterized in this study and they provide rare examples of HF coordinated to xenon(VI).

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.

Valence-Shell Electron-Pair Repulsion Theory Revisited: An Explanation for Core Polarization

Valence-shell electron-pair repulsion (VSEPR) theory constitutes one of the pillars of theoretical predictive chemistry. It was proposed even before the advent of the concept of "spin", and it is still a very useful tool in chemistry. In this article we propose an extension of VSEPR theory to understand the core structure and predict core polarization in the main-group elements. We show from first principles (Electron Localization Function analysis) how the inner- and outer-core shells are organized. In particular, electrons in these regions are structured following the shape of the dual polyhedron of the valence shell (3^rd period) or the equivalent polyhedron (4^th and 5^th periods). We interpret these results in terms of "hard" and "soft" core character. All the studied systems follow this trend, providing a framework for predicting electron distribution in the core. We also show that lone pairs behave as "standard ligands" in terms of core polarization. The predictive character of the model was tested by proposing the core polarization in different systems not included in the original set (such as XeF₄ and [Fe(CN)₆ ]^3- ) and checking the hypothesis by means of a posteriori calculations. From the experimental point of view, the extension of VSEPR to the core region has consequences for current crystallography research. In particular, it explains the core polarization revealed by high resolution X-ray experiments.

Matrix-Isolation and Quantum-Chemical Analysis of the C3v Conformer of XeF6, XeOF4, and Their Acetonitrile Adducts

A joint experimental-computational study of the molecular structure and vibrational spectra of the XeF₆ molecule is reported. The vibrational frequencies, intensities, and in particular the isotopic frequency shifts of the vibrational spectra for ¹²⁹XeF₆ and ¹³⁶XeF₆ isotopologues recorded in the neon matrix agree very well with those obtained from relativistic coupled-cluster calculations for XeF₆ in the C_3v structure, thereby strongly supporting the observation of the C_3v conformer of the XeF₆ molecule in the neon matrix. A C_3v transition state connecting the C_3v and O_h local minima is located computationally. The calculated barrier of 220 cm^-1 between the C_3v minima and the transition state corroborates the experimental observation of the C_3v conformer and the absence of the O_h conformer in solid noble gas matrices. For comparison matrix-isolation spectra have also been recorded and analyzed for the ¹²⁹XeOF₄ and the ¹³⁶XeOF₄ isotopologues. The matrix-isolation complexation shifts obtained for the XeF₆·NCCH₃ relative to those of free matrix isolated XeF₆ and CH₃CN are in good agreement with those reported for crystalline XeF₆·NCCH₃.

How to Twist, Split and Warp a σ-Hole with Hypervalent Halogens

Halogen bonds (XB) are no longer newcomers in the chemistry family. However, XB in hypervalent halogens has not been thoroughly studied. We provide a molecular orbital explanation of the shape and strength of XBs in hypervalent halogens and other species, focusing on the charge transfer and electrostatic aspects of these bonds. Our results show that σ-holes (and subsequently the XBs associated with them) can be easily divided and bent by the influence of equatorial substituents. The inductive effect of both the equatorial and axial groups can affect these distortions, but also the angle between the equatorial ligands has a large influence on the shape of the σ-holes and the molecular orbitals acting as electron acceptor. Although the observation of these warped XB can be hindered by other noncovalent interactions, they may be ubiquitous in crystal structures of hypervalent species, where multiple XB can appear as secondary interactions on each halogen. We propose what can be considered the archetypal hypervalent halogen donor (a pincer type iodosodilactone) and a Lewis dot structure that includes the σ-holes.

Synthesis and Characterization of [XeOXe](2+) in the Adduct-Cation Salt, [CH3 CN- - -XeOXe- - -NCCH3 ][AsF6 ]2

Acetonitrile and [FXeOXe- - -FXeF][AsF6 ] react at -60 °C in anhydrous HF (aHF) to form the CH3 CN adduct of the previously unknown [XeOXe](2+) cation. The low-temperature X-ray structure of [CH3 CN- - -XeOXe- - -NCCH3 ][AsF6 ]2 exhibits a well-isolated adduct-cation that has among the shortest Xe-N distances obtained for an sp-hybridized nitrogen base adducted to xenon. The Raman spectrum was fully assigned by comparison with the calculated vibrational frequencies and with the aid of (18) O-enrichment studies. Natural bond orbital (NBO), atoms in molecules (AIM), electron localization function (ELF), and molecular electrostatic potential surface (MEPS) analyses show that the Xe-O bonds are semi-ionic whereas the Xe-N bonds may be described as strong electrostatic (σ-hole) interactions.