Ionic organoboranes. Part 7. Molecular orbital study of electron distribution in the [7.10]hemiousenide ion; explanation for the anomalous B NMR spectrum

The ¹¹B NMR spectrum of the [7.10²]hemiousenide ion (C₇H₆B₁₀H₉⁻) demonstrates ground state +T cage to ring electron donation from the region of the two apical borons. The chemically non-equivalent apical borons give an apparent singlet NMR absorption and thus appear to be in very similar electronic environments, even though they are unsymmetrically arranged relative to the cationic ring. Ab initio 3-21G(∗) and semi-empirical PM3 calculations suggest that the π-system of the ring only interacts with cage molecular orbitals that are symmetrical relative to both apical borons in B₁₀H₁₀²− and that the electron density about the two apical borons is essentially identical in the [7.10²]hemiousenide ion. Significant ground state electron donation from cage to ring is observed, consonant with conclusions from previous polarographic studies.