During the early days of borane chemistry B5H9 was considered to be the stable pentaborane and although a BH vertex is isolobal with a naked P vertex, it is only recently that the first nido-five-vertex compounds with a naked phosphorus atom at the apical position were discovered. The most recent example is a valence isomer of the hitherto unknown triphosphacyclopentadienyl cation. These results shed light on the relative stability of planar and three-dimensional Cp+ derivatives.