Understanding The Electronic Structure And Stability Of B_nX_n^0/2- And Al₄X₄（n=4,6;X=H,F,Cl,Br,I,At,Ts） Clusters

Boron materials are widely used in many fields,such as semiconductor electronic components in aerospace,hydrogen storage materials and so on.Boron atom has a special skeleton structure and electron counting rules due to the lack of electrons.Since the first preparation of boranes by Alfred Stock,the enthusiasm for the study of borane clusters has been aroused.Borane clusters and their derivatives have attracted extensive attentions in inorganic chemistry due to their fascinating configurations and bonding patterns.Previous studies have found that the structure of boron clusters becomes more diverse and the bonding mode becomes more complex with the increase of their size.Therefore,we systematically studied the special electronic structure,stability and bonding mode of borane clusters starting from small clusters based on theoretical calculations.In general,elements in the same main group in the periodic table have the same number of valence electrons,Al and B are in the same main group and contain the same number of valence electrons,but the chemical properties are very different.Notably,Al₄H₄ was successfully synthesized and its stable configuration was proved to be Td symmetric.So,we will discuss Al₄X₄ as a B₄X₄ analogue.1.We find that the electronic structure and stability of B_nX_n^0/2-（n=4,6;X=H,F,Cl,Br,I,At,Ts）clusters can be tuned by the size of the boron cage and electronegativity of the ligand.Fragment orbital energy matching is of great importance to the orbital overlap and covalency of the cluster.The bonding interaction is mainly dominated by the ionic electrostatic interaction that decreases as the ligand becomes heavier,while covalency determined by the orbital interaction increases accordingly,attributing to the reducing electronegativity.Theσ-donations from the ligand to the boron cage as well as the nc-2e bonds contribute to theσaromaticity and superb stability.This work reveals the main factors that determine the structure and stability of boron clusters,which could give insights into the nature of chemical bonding for species with boron cages and even bulk boron.2.The electronic structure,stability and bonding characteristics of Al₄X₄（X=H,F,Cl,Br,I,At）were investigated by DFT calculation.According to our results,the charge distribution,HOMO-LUMO gap and stability of Al₄X₄ system are related to the energy matching degree of 1s-H/np-X and 3p-Al AOs with strong orbital overlap,which is similar to that of B₄X₄ cluster.In addition,fragment analysis shows that the（n+1）s-X AOs of H and light halogen ligand（F）also have important effects on the electronic structure of Al₄X₄.With the change of the halogen ligand from F to At,the Al₄X₄ interaction decreases gradually and the stability decreases.Due to the decrease of the electronegativity of the X atom and the enhancement of the orbital overlap,the covalence of the cluster increases gradually from F to At.The main components of the orbital interaction from ligand valence orbitals to the Al₄σ（r）_p orbitalsσ-donations.However,compared with B₄X₄ system,the stability and bonding strength of Al₄X₄ decrease,which may be caused by the low matching degree between 3p-Al AOs and the valence orbitals of the ligand because the energy of 3p-Al AOs is higher than that of 2s-B AOs.