Ab Initio Study of the Eight-Vertex closo-Heteroboranes X₂B₆H₆ (X = CH, SiH, GeH, SnH, and PbH). Is the Hexagonal Bipyramid a Viable Alternative?

Abstract

The hexagonal bipyramidal arrangement is calculated to be much closer in energy to the bisdisphenoid skeleton for the eight-vertex closo-heteroboranes with SiH, GeH, SnH, and PbH as the heteroatoms as compared with CH. The compatibility of the orbitals in overlap and the concept of the topological charge stabilization is extended to explain the structure and stability of these molecules. The qualitative approach breaks down for the heavier congeners of the carbon family as revealed by quasirelativistic pseudopotential calculations.

Three-Dimensional Aromaticity in Polyhedral Boranes and Related Molecules

“…For the 8-vertex E 2 B 6 H 8 isomers (E = C, Si, Ge, Sn, Pb), the most stable isomer in all cases is the 1,7-bisdisphenoidal isomer with the heteroatoms at nonadjacent degree 4 vertices. However, a hexagonal bipyramidal isomer of Si 2 B 6 H 8 with the silicon atoms at the degree 6 axial vertices is found to be only 17.2 kcal/mol higher in energy than the most stable 1,7-bisdisphenoidal isomer . For the icosahedral heteroboranes E 2 B 10 H 12 , the relative isomer stabilities remain antipodal (1,12) > nonadjacent (1,7) > adjacent (1,2) when carbon is replaced by silicon, although the energy differences are significantly smaller for the silicon derivatives …”

“…However, a hexagonal bipyramidal isomer of Si 2 B 6 H 8 with the silicon atoms at the degree 6 axial vertices is found to be only 17.2 kcal/mol higher in energy than the most stable 1,7bisdisphenoidal isomer. 124 For the icosahedral heteroboranes E 2 B 10 H 12 , the relative isomer stabilities remain antipodal (1,12) > nonadjacent (1,7) > adjacent (1,2) when carbon is replaced by silicon, although the energy differences are significantly smaller for the silicon derivatives. 125 McKee, Bu ¨hl, Charkin, and Schleyer used Gaussian methods at the MP2/6-31G* level to study fourcenter two-electron (4c-2e) bonding in deltahedral boranes and carboranes.…”

“…Jemmis and collaborators found through ab initio computations using Gaussian 90 and 94 that the relative isomer stabilities of some deltahedral heteroboranes change drastically upon substitution of carbon by silicon. − In the case of octahedral E 2 B 4 H 6 , the antipodal (1,6) carborane (E = C) is found to be more stable than the adjacent (1,2) isomer, whereas these relative stabilities are reversed for the silicon analogues (E = Si) . Similarly, for the 7-vertex pentagonal bipyramidal E 2 B 5 H 7 , the relative stabilities for the carborane isomers (E = C) are nonadjacent (2,4) > adjacent (1,2 and 2,3) > antipodal (1,7) whereas the stability sequence for the corresponding silaborane isomers is exactly the opposite …”