Neon compounds
Neon compounds are
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
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
Ligands
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
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
High pressure Van der Waals solids include (N2)6Ne7.[25]
Neon hydrate or neon clathrate, a
Neon atoms can be trapped inside
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
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]
Ions
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
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
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
The Ne*
2 molecule exists in an excited state in an
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
Bokiy's crystallochemical classification of minerals included "compounds of neon" as type 82. However, no such minerals were known.[52]
Predicted compounds
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]
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