The first three-membered P2B heterocycle, 1,2-di-tert-butyl-3-di-iso-propvlamino-1,2,3- diphosphaborirane (1,2-di-tert-butyl-3-di-iso-propylamino-1,2-diphospha-3-bora-cyclopro-pane) (1), was synthesized by the reaction of K(t-Bu)P-P(t-Bu)K with (i-Pr)2NBCl2 in n-hexane at -40 °C. 1 is stable at room temperature under inert conditions. The hetero-cyclophosphane 1 was characterized by elemental analysis, cryoscopic molecular mass determination, mass, IR, and NMR spectroscopy. The corresponding results confirm the monomeric structure with a cyclic P2B skeleton. No indication for a dimerization of 1 could be observed between +50 and -140 °C. Further information about the structure of 1 was obtained by temperature dependent 31P and 13C NMR spectra. Accordingly the t-butyl groups are situated on both sides of the P2B ring. Furthermore the arrangement at the boron atom is pyramidal due to the small endocyclic angle in the P2B ring. In consequence two configurations 1a and 1b are formed which undergo rapid exchange by inversion at the boron atom above ≈−100 °C. The 31P high field chemical shift is not consistent with a notable π back-donation from phosphorus to boron in the diphosphaborirane.