3c/4e -type long-bonding competes with ω-bonding in noble-gas hydrides HNgY (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I): a NBO/NRT perspective

Abstract

Noble-gas hydrides HNgY are frequently described as a single ionic form (H-Ng)(+)Y(-). We apply natural bond orbital (NBO) and natural resonance theory (NRT) analyses to a series of noble-gas hydrides HNgY (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I) to gain quantitative insight into the resonance bonding of these hypervalent molecules. We find that each of the studied species should be better represented as a resonance hybrid of three leading resonance structures, namely, H-Ng(+ -):Y (I), H:(- +)Ng-Y (II), and H^Y (III), in which the "ω-bonded" structures I and II arise from the complementary donor-acceptor interactions nY → σ*HNg and nH → σ*NgY, while the "long-bond" ([small sigma, Greek, circumflex]-type) structure III arises from the nNg → [small sigma, Greek, circumflex]*HY/[small sigma, Greek, circumflex]HY interaction. The bonding for all of the studied molecules can be well described in terms of the continuously variable resonance weightings of 3c/4e ω-bonding and [small sigma, Greek, circumflex]-type long-bonding motifs. Furthermore, we find that the calculated bond orders satisfy a generalized form of "conservation of bond order" that incorporates both ω-bonding and long-bonding contributions [viz., (bHNg + bNgY) + bHY = bω-bonding + blong-bonding = 1]. Such "conservation" throughout the title series implies a competitive relationship between ω-bonding and [small sigma, Greek, circumflex]-type long-bonding, whose variations are found to depend in a chemically reasonable manner on the electronegativity of Y and the outer valence-shell character of the central Ng atom. The calculated bond orders are also found to exhibit chemically reasonable correlations with bond lengths, vibrational frequencies, and bond dissociation energies, in accord with Badger's rule and related empirical relationships. Overall, the results provide electronic principles and chemical insight that may prove useful in the rational design of noble-gas hydrides of technological interest.

What is NBO analysis and how is it useful?

“…Stabilisation of the triad therefore depends solely on delocalisation of the on-axis Ng 'donor' lone pair (s-type for He; p-type for Ne, Ar, Kr, Xe, Rn) into the remaining vacant 'acceptor' NBO of proper symmetry,r XY for He orr à XY for Ne, Ar, Kr, Xe, Rn. [59] further confirm its importance in correlations, conservation laws, and other bonding regularities throughout the HNgY family. In the longbonding motif (4b), in contrast, resonance-type n Ng →r à XY =r XY delocalisation provides practically all triad stabilisation.…”

“…The NBO/NRT results of Zhang et al [59] paint a very different picture. Their work builds on previous NBO-based studies of Ng interactions ( [15], p. 299ff) that emphasise the importance of resonance-type 3c/4e (3-centre/4-electron) interactions in general XNgY chemistry.…”

“…The comprehensive NBO/NRT study of [59] extends to all 24 HNgY (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I) species, seeking to gain a consistent conceptual overview of the entire family. The results clearly demonstrate the necessity of full 3-resonance (I, II, III) description in order to consistently represent the trends in structural, vibrational, and energetic properties for both Ng and Y variations throughout the entire HNgY family.…”

“…[15,59,66]) as well as in correlative-type studies to be described below (Section 3.3). [15,59,66]) as well as in correlative-type studies to be described below (Section 3.3).…”

Theoretical investigation on the structure and stability of some neutral noble gas compounds containing Xe‐Xe bond

“…Although just limited noble gas molecules have been synthesized experimentally, many types of noble gas compounds are possible from theoretical point of view . This list includes compounds which have been also observed experimentally; such as XeF 6 , KrF 2 , HArF, clathrates and clathrate hydrates, coordination compounds, hydrates (Xe.6H 2 O), HeH + , oxyfluorides (XeOF 2 , XeOF 4 , Xe 2 O 3 F 2 , XeO 2 F 4 ), and fullerene compounds.…”