The Development Of Sigma-type Resonance Bonding Model And Its Application In High-energy Properties Of Noble-gas Hydrides

The hyperconjugative interactions occur between Sigma-type orbitals may cause Sigma-type resonance bonding in molecules.Developing the Sigma-type resonance bonding model is of great significance for enriching chemical bond theory and understanding noble-gas chemistry.The Nature Bond Orbital（NBO）and Nature Resonance Theory（NRT）analyses indicate that,the Sigma-type resonance bonding model involving two importantω-type resonance structures（B:M-X and（B-M）⁺:X^-）is appropriate for B???MX（B=NH₃,PH₃,H₂O,H₂S;M=Cu,Ag,Au;X=F,Cl,Br,I）complexes well,and the sum of B:M-X and（B-M）⁺:X^-resonance weightings is close to 100%.What’s more,the accurately preserved bond order conservation principle(b_B-M+b_M-X≈1)is obviously suitable for the studied B???MX complexes due to the special three-centers-four-electrons（3c/4e）Sigma-type resonance bonding.And when the rule of bond order conservation is added to,the rationalization of the ligand effect in B???MX complexes is found.Recently,the experimental studies show that,the distance between Lewis base B and metal atom M increase with the atomic number of the halogen atom X,both in C_3vv geometrical complexes H₃N···MX/H₃P···MX,and in C_S geometrical complexes H₂O···MX/H₂S···MX.On the basis of the accurately preserved bond order conservation principle b_B-M+b_M-X≈1,the B-M bond orders b_B-M-M should be decreased with increasing the atomic number of the halogen atom X due to the M-X bond orders b_M-X-X increase with the halogen X atomic number.There is no doubt that the bond lengths R_B-M-M would be increased with the atomic number of the halogen,and the corresponding tendency would exist in the bond dissociation energies?E_B-M.In other words,the rule of bond order conservation could be the key to explaining the ligand effects.All in all,this study developed the 3c/4e Sigma-type resonance bonding model,which helps us to understand the special physicochemical properties of Cu/Ag/Au containing complexes,and promote their application in medicine,catalysis and other fields.In addition,our group previous study found that the 3c/4e Sigma-type resonance bonding model in the B???MX system is also applicable to the noble gas and noble metal halide NgMX（Ng=Ar,Kr,Xe）.Due to the uniqueness of the rare gas atoms involved in the bonding,the synthesis and characterization of NgMX molecules have attracted much attention.What’s similar is that the observation and characterization of the rare gas hydride molecule HNgX turned the rare gas chemistry into an important new chapter.It is worth mentioning that all of the observed noble-gas hydrides HNgX are highly metastable species,whose two-body decomposition process HNgX→Ng+HX is always strongly exoergic,and that they represent high-energy materials.Therefore,we try to extend the concept of 3c/4e Sigma-type resonance bonding model into noble-gas hydrides HNg Y and noble-gas halogenides XNg Y（Ng is noble-gas atom,such as He,Ar,Kr,Xe,Rn;Y is electronegative fragment,such as CN/NC or CO⁺/CS⁺/OSi⁺）.As expected,the result for them is similar to that for NgMX species,the 3c/4e Sigma-type resonance bonding model is applicable to HNgY and XNgY molecules as well.What’s more,we find a novel resonance long-bonding structure H/X^?Y should be considered into HNgY and XNgY molecules apart from their twoω-type resonance structures H^-/X^-:⁺Ng-Y and（H/X-Ng）⁺:Y^-.It is worth mentioning that,there is a strong correlation between the highly exothermic two-body dissociation channel（HNgY→HY+Ng）and the long-bond order b_H-A.And the strength of long-bond is highly tunable for these noble-gas hydrides due to its dependence on the nature of the electronegative fragments Y,or the central noble-gas atom Ng.In general,the long-bond orders tend to decrease in the order He>Ar>Kr>Xe>Rn,while the b_H-A-A values tend to increase with the electronegativity Y fragments.Finally,we have also extended our research system to noble-gas halogenides XNgY,and studied the modulation of resonance bonding and stability in XNgY molecules by changing the halogen atom X,the noble-gas atom Ng,or the electronegative fragment Y.Overall,this study provides chemical insights into understanding and modulating high-energy property of noble-gas hydrides,and it is helpful for the design of HNgY molecules as energetic materials.