The relation between polyhedral borane sandwiches and endohedral complexes; the electronic structure and stability of X@Y_mB_nH_n+mq (X = He, Ne, Li, Be; Y = B, C, Si; m = 0-3; n = 12-9; q = -2 to +2), (C₂B₄H₆)₂X^q (X = Li, Al, Si; q = -3, -1, 0) and X₂@B₁₇H₁₇^q (X = He, Li; q = -2, 0)

Abstract

An electronic structural connection is established for sandwich complexes and polyhedral boranes containing encapsulated atoms. The charge requirements of these extreme geometrical patterns, examples 3 and 9, depend on the size of the central atom or on the distance between the adjacent rings. While going from the endohedral to the corresponding sandwich complexes the unoccupied a_2u and e_g molecular orbitals are stabilized considerably requiring additional 6 electrons for stability. The two endohedral atoms in the doped structures 10 resulting from the multidecker sandwich complexes 4 are found to stabilize the large borane skeleton. The energetics and geometries of the relatively less explored endohedral boranes show that endohedral silaboranes are more stable than the endohedral carbaboranes. In general, when an atom is encapsulated in a borane cage, its skeletal bonds are elongated. The exo bonds are shortened due to the possible reduction in the torsional strain between the adjacent vertices. A comparison of the endohedral complexes with the corresponding exo isomers shows that encapsulation makes the system more strained.