Structural Form and Nonrigidity in 6, 7, 8, and 9‐Atom Polyhedral Boranes ‐ Molecular Orbital Calculations

Extended Huckel molecular orbital calculations and the Jahn‐Teller theorem provide a reasonable rationale for the solution‐state structures and the stereochemical nonrigidity of some closo borane anions. In the eight‐atom borane, B8H82−, there is a very small energy gap between the highest occupied and lowest unoccupied molecular orbitals for D2d, D4d, and C2v forms, all of which are observed either for the solid or the solution states. In contrast, these gaps are large in Oh‐B6H62−, D5h‐B7H72−, and D3h‐B9H92− and smaller in plausible alternative polyhedral forms of these last three ions which are much more stereochemically rigid than B8H82− and do not show structural ambivalence in solution. Approximate second‐order Jahn‐Teller considerations applied to these various ions, B6 through B9, identify the B8H82− ion as the least rigid in this set of closo‐polyhedral borane ions. Relatively large energy barriers to rearrangements are expected for D3h‐B5H52− and D3h‐B9H92− on the basis of degeneracies in the presumed most favorable transition states or reaction intermediates, C4v‐B5H52− and C4v‐B9H92−, respectively, in the idealized rearrangement paths for these two borane clusters. Since there is a degeneracy in C5v‐B11H112, the Cs form is a far more reasonable transition state or intermediate for rearrangements of the highly fluxional C2v‐B11H112− ion.