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Neon compounds
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Neon compounds
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Neon compounds are chemical compounds containing the element neon (Ne) with other molecules or elements from the periodic table. Compounds of the noble gas neon were believed not to exist, but there are now known to be molecular ions containing neon, as well as temporary excited neon-containing molecules called excimers. Several neutral neon molecules have also been predicted to be stable, but are yet to be discovered in nature. Neon has been shown to crystallize with other substances and form clathrates or Van der Waals solids.

Neon has a high first ionization potential of 21.564 eV, which is only exceeded by that of helium (24.587 eV), requiring too much energy to make stable ionic compounds. Neon's polarisability of 0.395 Å3 is the second lowest of any element (only helium's is more extreme). Low polarisability means there will be little tendency to link to other atoms.[1] Neon has a Lewis basicity or proton affinity of 2.06 eV.[2] Neon is theoretically less reactive than helium, making it the least reactive of all the elements.[3]

Van der Waals molecules

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Van der Waals molecules are those where neon is held onto other components by London dispersion forces. The forces are very weak, so the bonds will be disrupted if there is too much molecular vibration, which happens if the temperature is too high (above that of solid neon).

Neon atoms themselves can be linked together to make clusters of atoms. The dimer Ne2, trimer Ne3 and neon tetramer Ne4 have all been characterised by Coulomb explosion imaging. The molecules are made by an expanding supersonic jet of neon gas. The neon dimer has an average distance of 3.3 Å between atoms. The neon trimer is shaped approximately like an equilateral triangle with sides 3.3 Å long. However the shape is floppy and isosceles triangle shapes are also common. The first excited state of the neon trimer is 2 meV above the ground state. The neon tetramer takes the form of a tetrahedron with sides around 3.2 Å.[4]

Van der Waals molecules with metals include LiNe.[5]

More Van der Waals molecules include CF4Ne and CCl4Ne, Ne2Cl2, Ne3Cl2,[6] I2Ne, I2Ne2, I2Ne3, I2Ne4, I2NexHey (x=1-5, y=1-4).[7]

Van der Waals molecules formed with organic molecules in gas include aniline,[8] dimethyl ether,[9] 1,1-difluoroethylene,[10] pyrimidine,[11] chlorobenzene,[12] cyclopentanone,[13] cyanocyclobutane,[14] and cyclopentadienyl.[15]

Ligands

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Neon can form a very weak bond to a transition metal atom as a ligand, for example Cr(CO)5Ne,[16] Mo(CO)5Ne, and W(CO)5Ne.[17]

NeNiCO is predicted to have a binding energy of 2.16 kcal/mol. The presence of neon changes the bending frequency of Ni−C−O by 36 cm−1.[18][19]

NeAuF[20] and NeBeS[21] have been isolated in noble gas matrixes.[22] NeBeCO3 has been detected by infrared spectroscopy in a solid neon matrix. It was made from beryllium gas, dioxygen and carbon monoxide.[17]

The cyclic molecule Be2O2 can be made by evaporating Be with a laser with oxygen and an excess of inert gas. It coordinates two noble gas atoms and has had spectra measured in solid neon matrices. Known neon containing molecules are the homoleptic Ne.Be2O2.Ne, and heteroleptic Ne.Be2O2.Ar and Ne.Be2O2.Kr. The neon atoms are attracted to the beryllium atoms as they have a positive charge in this molecule.[23]

Beryllium sulfite molecules BeO2S, can also coordinate neon onto the beryllium atom. The dissociation energy for neon is 0.9 kcal/mol. When neon is added to the cyclic molecule, the ∠O-Be-O decreases and the O-Be bond lengths increase.[24]

Solids

[edit]

High pressure Van der Waals solids include (N2)6Ne7.[25]

Neon hydrate or neon clathrate, a clathrate, can form in ice II at 480 MPa pressure between 70 K and 260 K.[26] Other neon hydrates are also predicted resembling hydrogen clathrate, and those clathrates of helium. These include the C0, ice Ih and ice Ic forms.[26]

Neon atoms can be trapped inside fullerenes such as C60 and C70. The isotope 22Ne is strongly enriched in carbonaceous chondrite meteorites, by more than 1,000 times its occurrence on Earth. This neon is given off when a meteorite is heated.[27] An explanation for this is that originally when carbon was condensing from the aftermath of a supernova explosion, cages of carbon form that preferentially trap sodium atoms, including 22Na. Forming fullerenes trap sodium orders of magnitude more often than neon, so Na@C60 is formed, rather than the more common 20Ne@C60. The 22Na@C60 then decays radioactively to 22Ne@C60, without any other neon isotopes.[28] To make buckyballs with neon inside, buckminsterfullerene can be heated to 600 °C with neon under pressure. With three atmospheres for one hour, about 1 in 8,500,000 molecules end up with Ne@C60. The concentration inside the buckyballs is about the same as in the surrounding gas. This neon comes back out when heated to 900 °C.[29]

Dodecahedrane can trap neon from a neon ion beam to yield Ne@C20H20.[30]

Neon also forms an intercalation compound (or alloy) with fullerenes like C60. In this the Ne atom is not inside the ball, but packs into the spaces in a crystal made from the balls. It intercalates under pressure, but is unstable at standard conditions, and degases in under 24 hours.[31] However at low temperatures Ne•C60 is stable.[32]

Neon can be trapped inside some metal-organic framework compounds. In NiMOF-74 neon can be absorbed at 100 K at pressures up to 100 bars, and shows hysteresis, being retained till lower pressures. The pores easily take up six atoms per unit cell, as a hexagonal arrangement in the pores, with each neon atom close to a nickel atom. A seventh neon atom can be forced under pressure at the centre of the neon hexagons.[33]

Neon is pushed into crystals of ammonium iron formate (NH4Fe(HCOO)3) and ammonium nickel formate (NH4Ni(HCOO)3) at 1.5 GPa to yield Ne•NH4Fe(HCOO)3 and Ne•NH4Ni(HCOO)3. The neon atoms become trapped in a cage of five metal triformate units. The windows in the cages are blocked by ammonium ions. Argon does not undergo this, probably as its atoms are too big.[34]

Neon can penetrate TON zeolite under pressure. Each unit cell contains up to 12 neon atoms in the Cmc21 structure below 600 MPa. This is double the number of argon atoms that can be inserted into that zeolite. At 270 MPa occupancy is around 20% Over 600 MPa this neon penetrated phase transforms to a Pbn21 structure, which can be brought back to zero pressure. However all the neon escapes as it is depressurized.[35] Neon causes the zeolite to remain crystalline, otherwise at pressure of 20 GPa it would have collapsed and become amorphous.[35]

Silica glass also absorbs neon under pressure. At 4 GPa there are 7 atoms of neon per nm3.[35]

Ions

[edit]

Ionic molecules can include neon, such as the clusters Ne
mHe+
n where m goes from 1 to 7 and n from 1 to over 20.[36] HeNe+ (helium neonide cation) has a relatively strong covalent bond. The charge is distributed across both atoms.[37]

When metals are evaporated into a thin gas of hydrogen and neon in a strong electric field, ions are formed that are called neonides or neides. Ions observed include TiNe+, TiH2Ne+, ZnNe2+, ZrNe2+, NbNe2+, NbHNe2+, MoNe2+, RhNe2+, PdNe+, TaNe3+, WNe2+, WNe3+, ReNe3+, IrNe2+, AuNe+ (possible).[38]

SiF2Ne2+ can be made from neon and SiF2+
3 using mass spectrometer technology. SiF2Ne2+ has a bond from neon to silicon. SiF2+
3 has a very weak bond to fluorine and a high electron affinity.[39]

NeCCH+, a substituted acetylene, is predicted to be energetically stable by 5.9 kcal/mol, one of the most stable organic ions.[40]

A neon containing molecular anion was unknown for a long time. In 2020 the observation of the molecular anion [B12(CN)11Ne]− was reported. The vacant boron in the anions [B12(CN)11]− is very electrophilic and is able to bind the neon. [B12(CN)11Ne]− was found to be stable up to 50 K and lies significantly above the Ne condensation temperature of 25 K. This temperature is remarkably high and indicates a weak chemical interaction.[41]

Ionic clusters

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Metal ions can attract multiple neon atoms to form clusters. The shape of the cluster molecules is determined by repulsion between neon atoms and d-orbital electrons from the metal atom. For copper, neonides are known with numbers of neon atoms up to 24, Cu+Ne1-24. Cu+Ne4 and Cu+Ne12 have much greater numbers than those with higher number of neon atoms.

Cu+Ne2 is predicted to be linear. Cu+Ne3 is predicted to be planar T-shaped with an Ne-Cu-Ne angle of 91°. Cu+Ne4 is predicted to be square planar (not tetrahedral) with D4h symmetry. For alkali and alkaline earth metals the M+Ne4 cluster is tetrahedral. Cu+Ne5 is predicted to have a square pyramid shape. Cu+Ne6 has a seriously distorted octahedral shape. Cu+Ne12 has an icosahedral shape. Anything beyond that is less stable, with extra neon atoms having to make an extra shell of atoms around an icosahedral core.[42]

Neonium

[edit]

The ion NeH+ formed by protonating neon, is called neonium. It is produced in an AC electric discharge through a mixture of neon and hydrogen with more produced when neon outnumbers hydrogen molecules by 36:1.[43] The dipole moment is 3.004 D.[43]

Neonium is also formed by excited dihydrogen cation reacting with neon: Ne + H2+* → NeH+ + H[44]

Far infrared spectrum of 20Ne1H+[43] 20NeD+ 22NeH+ 22NeD+
Transition observed frequency
J GHz
1←0 1 039.255
2←1 2 076.573 2 067.667
3←2 3 110.022 1 647.026 3 096.706
4←3 4 137.673 2 193.549 4 119.997 2 175.551
5←4 5 157.607 2 737.943 2 715.512
6←5 3 279.679 3 252.860
7←6 3 818.232 3 787.075
8←7 4 353.075 4 317.643
9←8 4 883.686

The infrared spectrum around 3μm has also been measured.[45]

Excimers

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The Ne*
2 molecule exists in an excited state in an excimer lamp using a microhollow cathode. This emits strongly in the vacuum ultraviolet between 75 and 90 nm with a peak at 83 nm. There is a problem in that there is no window material suitable to transmit these short wavelengths, so it must be used in a vacuum. If about one part in a thousand of hydrogen gas is included, most of the Ne*
2 energy is transferred to hydrogen atoms and there is a strong monochromatic Lyman alpha emission at 121.567 nm.[46]

Cesium can form excimer molecules with neon CsNe*.[47]

A hydrogen-neon excimer is known to exist. Fluorescence was observed by Möller due to bound free transition in a Rydberg molecule of NeH*. NeH is metastable and its existence was proved by mass spectroscopy in which the NeH+ ion is neutralized and then reionized.[48] The spectrum of NeH includes lines at 1.81, 1.60 and 1.46 eV, with a small band at 1.57 eV.[49] The bondlength in NeH is calculated as 1.003 Å.[48]

A helium neon excimer can be found in a mixed plasma or helium and neon.[50]

Some other excimers can be found in solid neon, including Ne+
2O−
 which has a luminescence peaking around 11.65 eV, or Ne+
2F−
 luminescing around 10.16–10.37 eV and 8.55 eV.[51]

Minerals

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Bokiy's crystallochemical classification of minerals included "compounds of neon" as type 82. However, no such minerals were known.[52]

Predicted compounds

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Analogously to the known ArBeO and the predicted HeBeO (beryllium oxide noble gas adducts), NeBeO is expected to exist, albeit with a very weak bond dissociation energy of 9 kJ/mol. The bond is enhanced by a dipole-induced positive charge on beryllium, and a vacancy in the σ orbital on beryllium where it faces the neon.[53]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Neon compounds

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Neon compounds refer to incorporating the (Ne), which is renowned for its exceptional chemical inertness arising from a closed-shell with a full octet of valence electrons. This inertness renders neon largely unreactive under ambient conditions, with no stable molecular compounds observed at . However, under extreme environments such as cryogenic matrix isolation or gas-phase ion trapping, a handful of transient neon-containing molecules and ions have been experimentally characterized, primarily featuring weak dative or van der Waals interactions rather than robust covalent bonds, including weakly bound neutrals like Ne...HF complexes verified by . Theoretical studies in the early predicted neutral neon hydrides and halides such as HNeF, HNeCl, and HNeBr, with dissociation energies below 10 kcal/mol, but these remain unisolated experimentally beyond computational verification. More recent advances include the 2020 observation of the first -bearing molecular anion, [B₁₂(CN)₁₁Ne]⁻, synthesized in a mass spectrometer by reacting superelectrophilic [B₁₂(CN)₁₁]⁻ with atoms at temperatures up to 50 K, demonstrating a boron- dative bond with a of approximately 2 kcal/mol. Theoretical studies predict additional neon compounds, such as NeF⁻, NeH⁻, and FNeO⁻, often as weakly bound anions or high-pressure phases, but experimental isolation remains elusive beyond gas-phase detections. Neon chemistry thus highlights the limits of reactivity, with ongoing research focusing on superelectrophilic partners and computational modeling to explore potential applications in exotic materials or .

Weakly Bound Neutral Compounds

Van der Waals molecules

Van der Waals molecules of neon are weakly bound neutral complexes primarily stabilized by London dispersion forces, which originate from correlated fluctuations in the densities of the constituent atoms, leading to induced interactions. These forces dominate due to neon's closed-shell electronic structure and lack of permanent dipoles, resulting in bond energies typically below 1 kcal/mol (approximately 350 cm⁻¹), far weaker than covalent or ionic bonds. The interaction potential for such systems is often modeled using Lennard-Jones or more advanced methods, revealing shallow potential wells with equilibrium distances around 3–4 Å. Homonuclear neon clusters exemplify these interactions, with the neon dimer (Ne₂) featuring a linear and a dissociation of about 3.6 cm⁻¹, as determined from high-level quantum chemical calculations. The neon trimer (Ne₃) adopts an isosceles triangular or pyramidal structure in its , with bound states supporting vibrational modes observable in experiments, while the tetramer (Ne₄) forms a tetrahedral configuration to minimize repulsion. These clusters' geometries and energies have been characterized through variational methods solving the on accurate surfaces. Heteronuclear van der Waals molecules involving neon include complexes such as CF₄·Ne, where the neon atom binds to the tetrafluoromethane molecule via dispersion forces, with interaction energies around 100–200 cm⁻¹ computed using second-order Møller-Plesset perturbation theory. Similarly, CCl₄·Ne exhibits anisotropic interactions influenced by the quadrupolar field of CCl₄, leading to preferred linear or T-shaped geometries, as probed by charge density analyses. Other notable examples are Ne₂Cl₂, a four-atom cluster with a total binding energy of approximately 146 cm⁻¹, and I₂·Neₓ (x=1–5) clusters, where multiple neon atoms solvate the iodine molecule in a cage-like arrangement, with stepwise binding energies decreasing from 50 cm⁻¹ for the first neon. Organic adducts like aniline·Ne form nearly perpendicular structures, with the neon positioned near the aromatic ring to maximize dispersion overlap. These complexes are experimentally detected using matrix isolation techniques, where or rare gas matrices at cryogenic temperatures (typically 4–20 K) trap the species for or , revealing van der Waals stretching modes. Gas-phase methods, such as supersonic jet expansion combined with or high-resolution UV , allow observation of rotational and vibrational transitions, confirming structures through measurements or predissociation linewidths. Due to their low binding energies, neon van der Waals molecules exhibit short lifetimes at room temperature, dissociating rapidly via thermal activation, with mean lifetimes on the order of picoseconds in isolated conditions. They remain stable under ultracold conditions or in dilute gases, and their dynamics are relevant to processes in the upper atmosphere, where neon participates in transient clustering during collisions.

Clathrate and trapped solids

Neon forms clathrate hydrates with under high-pressure and low-temperature conditions, where atoms occupy cavities within the hydrogen-bonded lattice without forming chemical bonds. The is typically cubic II (sII), with encapsulated in both pentagonal dodecahedral and hexakaidecahedral cages, yielding an ideal of Ne·5.67H₂O. These hydrates form at pressures above 200 MPa and temperatures below 280 K, with stability extending to about 1.9 GPa before undergoing pressure-induced amorphization. At higher pressures up to 600 GPa, computational searches predict additional stable phases, including dense neon-water compounds beyond traditional clathrates. Neon can also be trapped within fullerene cages through high-pressure insertion or molecular surgery techniques, resulting in endofullerenes such as Ne@C₆₀ and Ne@C₇₀. High-pressure methods expose open-cage fullerenes to neon gas at elevated temperatures and pressures, achieving insertion yields around 0.1%, while molecular surgery involves stepwise to enclose neon, enabling macroscopic quantities and characterization via and ¹³C NMR spectroscopy. Similar trapping occurs in (C₂₀H₂₀), where neon is inserted under , forming Ne@C₂₀H₂₀, with release upon heating due to the reversible van der Waals interactions. In metal-organic frameworks (MOFs), is adsorbed into porous structures like ammonium iron and formates under moderate pressure. At 1.5 GPa, neon is incorporated into [NH₄][Fe(HCOO)₃] and [NH₄][Ni(HCOO)₃], forming inclusion compounds Ne·NH₄Fe(HCOO)₃ and Ne·NH₄Ni(HCOO)₃, with size-selective trapping favoring neon over larger . Adsorption isotherms for other MOFs, such as and UiO-66, show neon uptake increasing with decreasing temperature (77–300 K) and pressure up to 1 bar, with capacities reaching several mmol/g at cryogenic conditions, highlighting potential for selective gas separation. Other trapped solids include the nitrogen-neon clathrate (N₂)₆Ne₇, a van der Waals insertion compound formed above 8 GPa at low temperatures, featuring distorted dodecahedral cages of molecules enclosing groups of 14 atoms. is also incorporated into lattices during freezing, as observed in accreted , where it is physically trapped within the at concentrations reflecting its in , stable under cryogenic conditions near 0 . These materials offer applications in neon storage and separation for cryogenic systems, leveraging reversible adsorption in MOFs and clathrates to achieve high volumetric capacities (up to 37 g/L at in some organic analogs) and low-energy release upon heating or pressure reduction.

Coordination Compounds

Neon as a ligand

functions as a weak sigma-donor in coordination chemistry, primarily with transition metals, through the formation of dative bonds involving donation from its to the metal center. These bonds are characterized by very low strengths, typically a few kJ/mol, reflecting the inert nature of and the resulting weak interactions. Key examples of such complexes include Cr(CO)5Ne, Mo(CO)5Ne, and W(CO)5Ne, which have been identified through in low-temperature neon matrices, where characteristic CO stretching frequencies shift due to the neon coordination. These species exhibit visible absorptions, such as Cr(CO)5Ne at 624 nm, facilitating their selective photoconversion with related xenon analogs. Additional instances involve , where coordinates to a center. The synthesis of these complexes typically involves the photolysis of metal hexacarbonyls, M(CO)6 (M = Cr, Mo, ), deposited in excess at cryogenic temperatures of 10-20 K, resulting in CO ligand substitution by neon from the matrix. For NeAuF, laser ablation of atoms in the presence of F2 within a neon matrix yields the species, confirmed by new IR absorptions in the Au-F stretching region. These neon-ligated complexes are highly labile, remaining stable only at low temperatures and dissociating upon warming above 20-30 , consistent with their weak bonding. Theoretical studies, including and coupled-cluster calculations, describe these interactions as primarily involving sigma donation with minimal backbonding, yielding Cr-Ne bond lengths around 2.5 Å and low vibrational frequencies indicative of the bond weakness. Such computations align with experimental IR data and highlight the role of matrix isolation in stabilizing these transient species.

Matrix-isolated complexes

Matrix-isolated complexes of involve neutral where atoms coordinate to electron-deficient centers, such as in or other systems, stabilized within solid matrices at cryogenic temperatures. These complexes are typically formed by co-depositing laser-ablated atoms with appropriate precursors, like molecular oxygen or , onto a cold substrate maintained at 4-10 , followed by thermal annealing or UV photolysis to promote reaction and isolation. The low temperature prevents diffusion and aggregation, allowing spectroscopic characterization of otherwise unstable . Bonding in these complexes arises from weak, primarily dispersive and σ-donation interactions between and the Lewis acidic center, with dissociation energies on the order of 1-2 kcal/mol. A prominent example is the beryllium oxide dimer complex coordinated by , such as the homoleptic Ne·Be₂O₂·Ne and heteroleptic variants Ne·Be₂O₂·Ar and Ne·Be₂O₂·Kr. These feature a cyclic Be₂O₂ core with atoms bridging the Be-O units, resulting in unusually short Be-Be distances (approximately 2.0 ) but no formal Be-Be bond, as confirmed by decomposition showing primarily electrostatic and charge-transfer contributions. The structures were synthesized by matrix isolation of atoms with O₂ in or mixed noble gas matrices, with identification via revealing characteristic O-Be stretches perturbed by coordination. Computational predictions from earlier studies, including those around 2014, anticipated such stable configurations, which were experimentally verified through these matrix techniques. Other notable neon-beryllium complexes include NeBeCO₃ and NeBeS. NeBeCO₃ forms upon co-deposition of beryllium atoms with O₂ and CO in a neon matrix, exhibiting a structure where neon binds to the electron-deficient beryllium in the BeCO₃ moiety, with the Ne-Be bond slightly longer and weaker than in analogous NeBeO (bond length ~2.1 Å, dissociation energy ~3 kcal/mol). Similarly, NeBeS arises from reactions of beryllium with H₂S in neon at 4 K, showing a linear Ne-Be-S arrangement stabilized by weak noble gas coordination. Infrared spectroscopy characterizes these via shifts in Be-X stretches (X = O, S, C), with the Ne-Be stretching mode appearing around 400 cm⁻¹, indicative of the weak interaction strength. These examples highlight neon's role in weakly binding to electron-deficient sites, distinct from stronger ligand interactions in gas-phase environments.

Ionic Compounds

Cationic neon species

Cationic neon species are positively charged ions in which acts as a bound to a central cationic center, primarily observed in the gas phase through techniques such as and . These species are typically formed via ion-molecule reactions, where a bare cation reacts with neutral atoms, or through in discharges and mass spectrometers. For example, the simple diatomic HeNe⁺ ion is produced by the reaction of He⁺ with Ne, resulting in a linear structure with a of 1.51 and a of approximately 0.717 eV. The vibrational of HeNe⁺ reveals a around 200 cm⁻¹, consistent with its weak bonding character, as determined from predissociation studies that observed over 100 rotational transitions in the A²Π–X²Σ⁺ system. Similarly, TiNe⁺ has been identified in neon matrices via electron-spin , exhibiting an octahedral coordination environment for the Ti⁺ center, indicative of weak electrostatic interactions typical of rare gas ligands. NeH⁺, a protonated species, serves as another simple example, though its detailed properties are discussed under neonium. These diatomic cations highlight neons role in stabilizing positive charges through polarization-induced bonds. More complex cationic species demonstrate neons ability to form solvation shells around metal centers. For instance, Cu⁺Neₙ clusters (n=1–24) exhibit stepwise solvation, with the first shell completing at n=12 in an , as evidenced by mass spectra showing enhanced stability at magic numbers n=4 and n=12, with binding energy steps of 0.18 eV and 0.13 eV, respectively. ZnNe₂⁺ represents a smaller complex where neon atoms coordinate to the zinc dication in a linear or bent configuration, contributing to overall cluster stability through van der Waals-like interactions augmented by charge-induced forces. These structures are linear or symmetric for small n, transitioning to polyhedral arrangements as n increases, and their formation occurs via sequential attachment in low-pressure ion traps or beams. Other notable complexes include SiF₂Ne₂⁺, formed by the reaction of SiF₃²⁺ with in a mass spectrometer, featuring neon-silicon bonds with dissociation energies around 1–2 eV, and NeCCH⁺, a bent with a C–H stretching frequency of 3101.9 cm⁻¹ and a significant dipole moment that aids potential detection. The stability of these species generally ranges from 1 to 5 eV per , making them transient under standard conditions but relevant to interstellar chemistry, where low densities and by cosmic rays facilitate their transient existence in diffuse clouds.

Anionic neon species

Anionic neon species represent an exceptionally rare class of neon compounds, as the inert nature of neon has long precluded the formation of stable anions incorporating the element. Unlike cationic neon species, which are more readily observed in gas-phase experiments, anionic counterparts require highly specialized conditions to overcome neon's reluctance to engage in bonding interactions. The first experimental observation of such a species occurred in , marking a significant milestone in noble gas chemistry. The key example is the molecular anion [B₁₂(CN)₁₁Ne]⁻, featuring neon bound to a boron cage derived from a cyanoborohydride framework. This compound arises from the interaction between neon and the superelectrophilic [B₁₂(CN)₁₁]⁻ anion, where the electron-deficient boron vertex acts as the binding site. The structure positions neon adjacent to this boron atom, situated in a "" formed by the surrounding negatively charged cyano groups, with a computed B–Ne bond length of 2.09 . Confirmation of the structure relied on detecting isotopic shifts with ²⁰Ne and ²²Ne, alongside and calculations using the B3LYP/def2-QZVPP method. Synthesis involves generating the precursor [B₁₂(CN)₁₂]²⁻ through a photochemical reaction of [B₁₂I₁₂]²⁻ with cyanide sources in the presence of sodium and reduced , followed by isolation of the [B₁₂(CN)₁₁]⁻ impurity anion. This anion is then introduced into a cryogenic Paul ion trap cooled to approximately 6 K, where it is exposed to a mixture of 10% in gas, facilitating the attachment of to form [B₁₂(CN)₁₁Ne]⁻. The bonding is characterized as a weak donor-acceptor interaction, dominated by dispersion and electrostatic contributions, with an attachment of -8.9 kJ/mol at 0 K, as determined by second-order Møller-Plesset (SCS-MP2) and energy decomposition analysis. The compound exhibits remarkable stability for a neon-containing anion, persisting in the ion trap up to 50 —well above neon's condensation temperature of 25 —before the signal diminishes due to thermal dissociation. This persistence challenges the of neon's inertness and suggests potential pathways toward more stable neon compounds, possibly through stabilization. The partial positive charge on neon (+0.11 e via natural population analysis) underscores the electrophilic nature of the interaction at the boron site. No other anionic neon species have been experimentally confirmed to date, though theoretical studies predict additional possibilities, such as neon binding to other superhalogen anions like [B₁₂X₁₁]⁻ (X = or ), which could exhibit stronger interactions under similar low-temperature conditions.

Ionic clusters

Ionic clusters of neon primarily involve mixed systems with , forming cations denoted as He_n Ne^+ where n ranges from 4 to over 20, with neon serving as the core solvated by helium atoms in inner shells, though structures with additional neon atoms in outer positions (up to m=7 in Ne_m He_n ^+) have been observed in more complex mixtures. These clusters arise from the weak interactions characteristic of , enabling the study of dynamics in ultracold environments. Formation of these clusters occurs through supersonic expansion of gas to generate nanodroplets, which are doped with atoms via pickup techniques, followed by using electron impact, with (10-30 eV), or charge transfer processes within the droplet. Ion trap further allows size selection and growth of clusters by sequential attachment of atoms. The resulting ions exhibit linear or ring-like geometries for small sizes, with the He-Ne^+ bond length at equilibrium around 1.51-1.81 and of 0.717 eV for HeNe^+, decreasing significantly for additional helium atoms (e.g., 0.06 eV for the second He). Bonding in He_n Ne^+ clusters is dominated by electrostatic attraction between the Ne^+ core and the induced s on helium atoms, supplemented by charge-induced dipole interactions that stabilize the shells. Mass spectra display at cluster sizes like He_4 Ne^+, He_11 Ne^+, and He_13 Ne^+, signaling enhanced stability due to completed shells and minimized energy. Experimental probes include threshold to determine binding energies and fragmentation pathways, as well as ion mobility measurements to infer structural asymmetries and compactness. These ionic clusters model solvation processes in noble gas matrices, providing insights into weakly bound systems relevant to cryogenic environments, and hold astrophysical interest for ion chemistry in dilute interstellar regions where noble gas clustering may influence radiative processes.

Exotic and Excited Neon Species

Neonium

NeH⁺, known as neonium, is a diatomic molecular ion consisting of a neon atom bonded to a hydrogen atom, serving as the neon analog of the helium hydride ion HeH⁺ and the trihydrogen cation H₃⁺ in its ability to form a stable protonated species through dative bonding. This ion represents one of the few known examples of neon forming a covalent-like bond, where the neon atom donates electron density to the proton. The ion is formed in hydrogen-neon plasmas or electric discharges, primarily through the proton-transfer reaction Ne + H₂⁺ → NeH⁺ + H, which is endothermic for low vibrational states of H₂⁺ but becomes favorable at higher energies. This process occurs in laboratory conditions such as hollow cathode discharges, where NeH⁺ is generated as a transient species in mixed gas environments. Structurally, NeH⁺ is a linear molecule with an equilibrium bond length of approximately 1.00 Å and a dissociation energy of about 2.3 eV, indicating a relatively weak but stable bond compared to typical covalent hydrides. The bonding can be described using molecular orbital theory, where the neon atom's lone pair in a 3s-like orbital donates electrons to the empty 1s orbital of the proton, resulting in a two-center dative bond. The spectrum of NeH⁺ features the fundamental vibrational band (v=1←0) for the Ne-H stretch near 2564 cm⁻¹ for the , enabling its identification through high-resolution . This was first observed in the in 1982 via using a difference frequency on ions produced in a hollow cathode discharge. Further confirmation and detailed characterization came from high-resolution emission in 2001, which provided accurate rotational-vibrational constants and supported function derivations. As of 2025, while NeH⁺ has been predicted to exist in interstellar environments and cometary plasmas due to its formation pathway, it has not yet been conclusively detected in space.

Excimers

Excimers are short-lived, metastable molecules formed exclusively in electronically excited states, where two atoms bind temporarily before dissociating upon return to the . The prototypical , Ne₂*, arises from the collision of an excited atom (Ne*) with a atom (Ne), typically through a three-body process: Ne* + 2Ne → Ne₂* + Ne. This formation is facilitated by impact excitation or electrical discharge in gas, where high-energy electrons populate the Ne*(³P) metastable state, which then associates with surrounding atoms under sufficient . Upon relaxation, Ne₂* emits vacuum ultraviolet (VUV) light from its bound excited potential curve to the repulsive , producing a broad continuum centered around 83 nm in the second band. The excited-state bonding is characterized by a shallow well with a dissociation energy of approximately 0.5 eV, enabling emission without stable ground-state persistence. Radiative lifetimes for these s range from 10 to 100 ns, reflecting rapid dissociation after release. Spectroscopic studies reveal multiple continua (first at ~73 nm, second at ~83 nm, and third beyond 100 nm), arising from different excited-state configurations accessed via discharge excitation. Heteronuclear neon excimers, such as CsNe* and NeH*, exhibit similar excited-state bonding and emission properties. CsNe* forms via association of excited cesium with , yielding emission bands from transitions between excited states, observed in alkali-noble gas mixtures. NeH* behaves as a Rydberg excimer radical, with bound excited states leading to in the VUV region, studied through matrix isolation and gas-phase . The heteronuclear HeNe* excimer has been implicated in mixed rare-gas discharges, contributing to weak VUV emission near 80 nm, though less characterized than homonuclear variants. Neon excimers power VUV lamps used in applications requiring short-wavelength radiation, including for patterning and surface disinfection in controlled environments. These lamps leverage the 83 nm emission for photochemical reactions and microbial inactivation, offering mercury-free alternatives with high efficiency in inert atmospheres.

Neon in Nature and Theory

Minerals

No neon minerals are recognized by the International Mineralogical Association (IMA), as neon's high volatility and chemical inertness prevent the formation of stable solid compounds under natural Earth surface or subsurface conditions. Despite theoretical possibilities for neon incorporation in mineral structures, such as clathrates, no distinct neon-bearing mineral species have been identified or classified. Trace amounts of have been detected in natural minerals like and , primarily trapped within fluid inclusions or lattice defects during formation. In , is released from fluid inclusions via crushing and analyzed by , revealing concentrations on the order of parts per million (ppm) in hydrothermal samples from ancient greenstone belts. Similarly, in , is trapped from mantle sources, with concentrations typically in the ppm range, detected through stepwise heating and . These occurrences highlight neon's role as a trapped rather than a structural component. Neon isotopes, including ²⁰Ne, ²¹Ne, and ²²Ne, provide geochemical insights into mantle-derived minerals such as those in kimberlites and basalts. Analyses of in these minerals show primordial signatures, with ²⁰Ne/²²Ne ratios approaching solar values (around 13.8), indicating retention of early solar system volatiles in the deep mantle. A 2025 study of kimberlites revealed primordial signatures, supporting deep mantle origins with ²⁰Ne/²²Ne ratios near solar values. Cosmogenic ²¹Ne, produced by nuclear reactions in the lattice, and atmospheric ²²Ne are distinguished through isotopic ratios measured via high-precision , revealing mixtures of mantle and surface influences. Such isotopic data from mantle xenoliths and inclusions trace volatile transport from the Earth's interior. Experimental analogs for natural neon trapping involve high-pressure synthesis to simulate conditions, where neon solubility in silicates or fluids is enhanced. Studies using diamond anvil cells have demonstrated neon incorporation into mineral-like phases at pressures exceeding 10 GPa, mimicking defect trapping observed in natural samples. These experiments, often employing , provide models for how neon might be retained in geological settings without forming distinct compounds. The presence of trace neon in minerals has significant implications for understanding Earth's volatile inventory and degassing history. By analyzing neon isotopes in mantle-derived samples, researchers infer the extent of primordial gas retention versus atmospheric exchange, contributing to models of planetary accretion and mantle evolution. These findings underscore neon's utility as a tracer for deep Earth processes, despite its scarcity in solid phases.

Predicted compounds

Theoretical studies have proposed several neutral neon-containing molecules that could exhibit weak chemical bonding, though none have been synthesized to date. Theoretical investigations using (DFT) have explored the structure of neon difluoride (NeF₂), suggesting it as a weakly bound with limited stability. Other neutral candidates include NeCl₂ and donor-acceptor complexes such as Ne–BF₃, where neon acts as a weak Lewis base interacting with electron-deficient boron centers; these are forecasted to have binding energies on the order of 5–10 kJ/mol, sufficient for isolation in noble gas matrices but prone to dissociation at higher temperatures. Advanced computational methods, including DFT and high-level ab initio approaches like coupled-cluster theory, have been instrumental in predicting the viability of these compounds. These techniques reveal that neon's high promotion energy and filled valence shell limit bond formation to weak interactions, often stabilized only under extreme conditions such as high pressure or cryogenic matrix isolation. For example, calculations indicate that NeO and related species may persist in neon matrices at temperatures below 10 , with potential energy surfaces showing shallow minima corresponding to van der Waals-like bonding. Donor-acceptor complexes like Ne–BF₃ are similarly predicted to form via charge-transfer mechanisms, with equilibrium geometries featuring neon positioned along the boron-fluorine axis. However, warming these systems is expected to trigger rapid decomposition due to the low binding energies and neon's inherent inertness, posing significant synthetic challenges.

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