The structure of stable pentaborane

Stable pentaborane, B5H9, is one of the boron hydrides, a short, comparatively little studied series of extraordinary compounds for which a satisfactory elementary valence theory is lacking. In 1947 we decided to undertake new electron diffraction studies of the molecular structures. The early diffraction work and most of the theoretical discussion had been too much influenced (it now seems) by unfortunate analogies to ordinary valence compounds, and it had become reasonably clear that at least the old, ethane-like structure for diborane was incorrect and that a bridge structure (I) was more likely.