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 can be considered to be correctly termed hypho-7,8-[S2B6H9]-.