3c/4e (Sigma)Over-Cap-Type Long-Bonding Competes With Omega-Bonding In Noble-Gas Hydrides Hngy (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I): A Nbo/Nrt Perspective

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 "omega-bonded'' structures I and II arise from the complementary donor-acceptor interactions n(Y) -> sigma(HNg)* and n(H) -> sigma(NgY)*, while the "long-bond'' ((sigma) over cap -type) structure III arises from the n(Ng) -> ((sigma) over cap (HY)*/((sigma) over cap (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 omega-bonding and ((sigma) over cap -type long-bonding motifs. Furthermore, we find that the calculated bond orders satisfy a generalized form of "conservation of bond order'' that incorporates both o-bonding and long-bonding contributions [viz., (b(HNg) + b(NgY)) + b(HY) = b(omega-bonding) + b(long-bonding) = 1]. Such "conservation'' throughout the title series implies a competitive relationship between o-bonding and ((sigma) over cap -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.