A computational analysis of the apparent nido vs. hypho conflict: are we dealing with six- or eight-vertex open-face diheteroboranes?

Abstract

A series of computational studies have been undertaken to investigate the electronic structures and bonding schemes for six hetero-substituted borane cages, all of which have been presented in the literature as potential hypho structures. The six species are hypho-7,8-[C₂B₆H₁₃]⁻ (1a), hypho-7,8-[CSB₆H₁₁]⁻ (1b), hypho-7,8-[S₂B₆H₉]⁻ (1c), hypho-7,8-[NSB₆H₁₁] (1d), exo-7-Me-hypho-7,8-[NCB₆H₁₂] (1e), and endo-7-Me-hypho-7,8-[NCB₆H₁₂] (1f) and the so-called mno rule has been applied to each of them. As no structural data are known for the carbathia-, azathia-, and dithiahexaboranes, we have also applied the ab initio/GIAO/NMR structural tool for 1b–1d, with 1c having been prepared for this purpose. We conclude that an mno count of 10 means that 1a, 1b, 1d, 1e, and 1f should be termed pseudo-nido or pseudo-hypho. Only 1c can be considered to be correctly termed hypho-7,8-[S₂B₆H₉]⁻.