Using the ab initio MP2/3-21G* method, the structure of B~4H~8X (X=Li^-, Be, B^+, C^2^+, B^-, C, N^+, BeH^-, BH and CH^+) and their fragments of B~4H~8 have been studied. The results show that terminal hydrogen positions are controlled by the acme cap (X) in B~4H~8X molecules. When the cap atoms have smaller electronegntivity and the ligand orbitals have more diffusion, terminal hydrogen atoms can be observed below the B~n plane (against the cap). In contrast, when terminal hydrogen atoms towards the cap (X), the bridging hydrogen atoms are away from it. Further investigation on dissociation energies and correlation energies of B~4H~8X was carried out. We have observed that the structure of 1 and 2 we are unstable while the structure of 3～10 were relatively stable. In all fragments of B~4H~8, the structure with 2112 configuration is most stable.

Key words: CHEMICAL BONDS, AB INITIO CALCULATION, BORANES, BONDING, CONFIGURATION, BINDING ENERGY, STRUCTURAL STABILITY, DISSOCIATION ENERGY, CONES, STRUCTURE CHARACTERISTICS