Theoretical Investigation On The Resonance Bonding Of Noble-gas Hypervalent Molecules

The history of noble-gas chemistry originates from the inspiring suggestions by Pauling,that the heavier noble-gas atom(for example, krypton and xenon) may form stable molecules with other atoms. Bartlett and co-workers observed the first noble-gas molecule XePtF6. Since then, more and more noble-gas molecules have been identified by experimental scientists.Among previous investigations, the preparation of the first experimentally known neutral argon molecule, HArF, by Khriachtchev and co-workers in 2000 is particularly noteworthy. Because the noble-gas molecules violate the octet rule, the bonding of noble-gas molecules has attracted much attention. Early work includes the energy decomposition analysis and some analyses about the possible resonance structures of HNgY. Recently, Weinhold and co-workers investigated the bonding of noble-gas difluoride NeF2 by using natural resonance theory. They proposed the three-center, four-electron long-bonding, and explained the bonding of NeF2. In this work, we investigate the bonding of HNgY from resonance bonding perspective. All the hypervalent molecules are fully optimized and characterized with Gaussian 09 program package at B3 LYP level of theory, and the aug-cc-pVTZ(-pp) basis set was adopted for all the atoms. We analyze the bonding of HNgY by using the natural bond orbital(NBO) and natural resonance theory(NRT). Furthermore, we also study the bonding of MNgY and NgMY, which include two noble elements(noble-gas and noble metal). The former has been observed in experiments and the latter has been identified by the theoretical studies. The main work and conclusions are as follows:(1)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 resonance structures, namely, H-Ng+-:Y(I), H:- +Ng-Y(II), and H^Y(III), in which the “?-bonded”structures I, II arise from complementary donor-acceptor interactions nY?s*HNgand nH?s*NgY,while the “long-bond”(?-type) structure III arises from nNg??*HY interaction. Furthermore, we find that the calculated bond orders satisfy a generalized form of “conservation of bond order”that incorporates both ?-bonding and long-bonding contributions [viz.,(bHNg + bNgY) + bHY =b?-bonding + blong-bonding = 1]. Such “conservation” throughout HNgY molecules implies a competitive relationship between ?-bonding and ?-type long-bonding. Based on the competitive relationship, we find the electronegativity of Y and outer valence-shell character of the central Ng atom effect the 3c/4e hyperbonding of HNgY. The calculated bond orders are also found to exhibit chemically reasonable correlations with bond lengths, vibrational frequencies, and bond dissociation energies. These results may be useful in rational design of noble-gas hydrides of technological interest.(2) We also perform the NBO/NRT analysis for the hypervalent molecules MNgY(M = Cu,Ag, Au; Ng = Ar, Kr, Xe, Rn; Y = F, Cl, Br, I). The NBO/NRT results indicate that the 3c/4e long-bonding also exists in this kind of hypervalent molecules. And MNgY should be better regarded as a resonance hybrid(M-Ng+-:Y, M:- +Ng-Y and M?Y), the including resonance structures are similar to the resonance structures of HNgY. The NRT bond orders in these hypervalent molecules still accord with the conversation of bond order.(3) Bonding analysis are performed for a series of NgMY(Ng = He, Ne, Ar, Kr, Xe, Rn;M=Cu, Ag, Au; Y=F, Cl, Br, I) by using natural bond orbital(NBO) and natural resonance theory(NRT) methods. The NBO/NRT results clearly reveal that each of the studied compounds should be better described as a resonance hybrid, arising mainly from complementary donor-accepter interactions nNg ? ?*MYand nY ? ?*NgM, which can be represented as Ng: M-Y Ng-M+-:Y.Detailed analyses of the NRT descriptors indicate that the NRT bond orders satisfy conversation of bond order, namely, bNgM + bMY ? 1, implying the competitive aspect of the resonance-type three center-four electron ?–bonding around the noble metal.