The bond dissociation energies (BDEs) of BH 3–Ng complexes are very low, which might be due to the requirement of large preparation energy by planar boron acceptors to achieve pyramidal conformation upon complexation with noble gases. Is there a possibility of formation of thermodynamically stable noble-gas compounds featuring donor–acceptor interactions without the use of steric protection? Is it possible for the noble gases to form a usual two-center–two-electron bond? To answer these questions, we have carried out quantum chemical calculations on donor–acceptor complexes between noble gases and group 13 elements ( Scheme 1). (9) Their calculations reveal that the dissociation process, A–Ng–D = Ng + AD (A = acceptor and D = donor), is endergonic (Δ G > 0), indicating that the cryptand-encapsulated noble-gas compounds are thermodynamically stable. have theoretically proposed the use of a rigid cage, such as a push–pull cryptand ligand, which contains both donor (D) and acceptor (A) sites. (12) To overcome the problem of dissociation, Mück et al.
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However, calculations reveal that both HArF and HKrF molecules are metastable due to exothermic dissociation into free Ng and HF. (9) Such a bonding situation can describe the experimentally synthesized compounds such as HArF (10) and HKrF, (11) which were prepared by the photolysis of HF in argon and krypton matrices.
(8) The other possibility of three-center–four-electron bond formation is through donor–acceptor interaction, as pointed by Mück et al. Similar bonding situation has also been described in Au +–Ng–F –. The bonding situation in the stable compound, NgF 2, is described as three-center–four-electron bonds, (7) where terminal fluorine atoms also participate in the bond formation. Noble-gas compounds are generally metastable and readily dissociate into their atomic form.
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