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

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](-).