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Just as $\ce{HArF}$ was synthesized...Could something like $\ce{Li-Ar-Li}$ exist with AXE geometry $\ce{AX2E3}$? Any other argon compound proposals out there?

I also know that there are some posible excimers like $\ce{LiAr}$ and $\ce{KrF}$ (maybe $\ce{KrF2}$). Can they be considered covalent compounds, even if explained via excited dimers/excimers or the OM theory (recall the technical name for lithium-doped argon matrices in the chemical literature)?

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ArBeO and ArBeS are experimentally known.

ArAuF, ArAgF and ArCuF are also experimentally known.

CUOAr$_n$ and related complexes are known.

ArBeNCN and ArBeNBO are predicted by theory.

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  • $\begingroup$ But, could be Li-Ar-Li (AX2E3, linear molecule, trigonal bipyramid orbital) possible or not? Theoretically, at least... $\endgroup$ – riemannium Jun 16 '14 at 15:23
  • $\begingroup$ Lithium is very electropositive, so it tends not to form covalent bonds. $\endgroup$ – DavePhD Jun 16 '14 at 15:50
  • $\begingroup$ Hydrogen is also electropositive...And we have HArF and, as well, excimer-like Li-Ar (but unstable) compounds. IN the Pauling scale, H is 2.3 and lithium 0.9. I mean, couldn't it be possible? I am not asking if it is hard (likely), but even if electronegativity is LOW, I can not see why it could not be possible in certain extreme conditions such a bond... Of course, lithium doped argon excimers are something different to my idea. $\endgroup$ – riemannium Jun 16 '14 at 17:45
  • $\begingroup$ And I understand, even if they have special features, ionic bonds like "very polar" covalent bonds...Theoretically, what would forbid a Li-Ar-Li "molecule"? Example: journals.aps.org/pr/abstract/10.1103/PhysRev.179.19 $\endgroup$ – riemannium Jun 16 '14 at 17:51
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    $\begingroup$ If the 2 elements being considered have an electronegativity difference of >1.7, a covalent bond will not form. The Allen electronegativity of Ar is 3.24 and of Li is 0.91. The references to LiAr you cite are van der vaals molecules not covalent molecules. Li-Li has an electronegativity different of zero. The sodium reference is Na+, not neutral Na, and says the same type of cluster forms for Li+ with Ar. $\endgroup$ – DavePhD Jun 16 '14 at 19:13
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Wikipedia now lists dozens (hundreds?) of known argon compounds, so the concept is nothing new. Most are a bit contrived, but one compound that could have a real existence on Earth -- or rather, in Earth, is a nickel-argon compound:

At 140 GPa and 1500K nickel and argon form an alloy, NiAr.[1] NiAr is stable at room temperature and a pressure as low as 99 GPa. It has a face-centred cubic (fcc) structure. The compound is metallic. Each nickel atom loses 0.2 electrons to an argon atom which is thereby an oxidant. This contrasts with Ni3Xe, in which nickel is the oxidant. The volume of the ArNi compound is 5% less than that of the separate elements at these pressures. If this compound exists in the core of the Earth it could explain why only half the argon-40 that should be produced during the radioactive decay that produces geothermal heating seems to exist on the Earth.[2]

There is an implication in the above passage that argon can assume a partial negative charge, despite ostensibly having full valence subshells, a feature not evident in the more usual noble gas compound choices such as oxides and fluorides.

Cited references:

1. Dalton, Louisa (30 October 2019). "Argon reacts with nickel under pressure-cooker conditions". Chemical & Engineering News. Retrieved 6 November 2019. Electronic link goes to Ref. 2.

2. Adeleke, Adebayo A.; Kunz, Martin; Greenberg, Eran; Prakapenka, Vitali B.; Yao, Yansun; Stavrou, Elissaios (15 October 2019). "A High-Pressure Compound of Argon and Nickel: Noble Gas in the Earth's Core?". ACS Earth and Space Chemistry. 3 (11): 2517–2524. Link

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