The stereoisomers 1a and 1b open the possibility to determine the contribution of the 3s and 3p character of phosphorus in a trigonal-bipyramidal (TBP) configuration. The ESR experiments clearly show that there is a small excess of 3s spin density in the equatorial ligand in comparison with the apical one. Although this result does not conflict with the physical and chemical properties of phosphorus in a TBP, it refines the established idea that the sp3d hybridization of phosphorus can be constructed from three equatorial sp2 ligands and two apical pd ligands. This study was carried out by starting from the precursor 2, present as a single crystal. By means of UV and X-ray irradiation the different stereoisomers could be obtained. The single-crystal experiments allowed us to obtain the directional information, i.e., location of the odd electron in the equatorial and apical location.