Stability of polyhedral borane anions and carboranes

Abstract

Polyhedral borane anions and carboranes that can be constructed formally from the interaction of rings and caps will be stable with six interstitial electrons. Interstitial electron count is obtained by summing the number of π electrons of the ring and the electrons of the caps involved in ring cap binding. Thus B₇H₇^-2 (D_5h) has 6 interstitial electrons (none from the B₅H₅ ring, two each from the twobh caps and two negative charge),mndo calculations on isoelectronic pyramidal molecules B₆H₆^-4 (C_5v), B₅H₅CH^-3 (C_5v), B₅H₅^-4 (C_4v), B₄H₄CH^-3 (C_4v), B₄H₄ ^-4 (T_d) and B₃H₃CH^-3 (C_3v) suggests a criterion based on the out-of-plane bendings of the ring B-H bonds to select the best combination of borocycles and BH or CH caps. Three-membered borocycle prefers CH cap, five-membered borocycle prefers BH cap. The preference of four-membered ring for BH or CH cap is not as pronounced. The extra stability of B₁₂H₁₂^-2 arises from the geometry of the icosahedron. The relative stabilities ofnido andcloso carboranes follow from these rules.