The stable phosphanylidene-σ 4 -phosphorane DmpP PMe 3 (1, Dmp=2,6-Mes 2 C 6 H 3 ) has been examined by single-crystal X-ray diffraction methods. The structure of 1 features a relatively short P P bond length of 2.084(2) 9. Reactions of 1 with various electrophiles demonstrate the nucleophilic behavior of the phosphanylidene atom of 1 and also provide access to new organophosphorus compounds. For example, addition of excess BH 3 (in the form of either BH 3 .THF or BH 3 .SMe 2 ) to 1 leads to formation of a mono-borane adduct DmpP(BH 3 )PMe 3 . Reactions of carbon and silicon based electrophiles EX (E=R 3 C or R 3 Si; X=halide or OTf - ) produce either diphosphanium salts [DmpP(E)PMe 3 ]X or phosphines DmpP(E)X. In some cases equilibrium mixtures of both product types are observed, and the equilibria can be shifted by addition of either X - or PMe 3 . Compound 1 is also readily protonated by HOTf, HCl and PhOH. As found for the carbon and silicon based electrophiles, the nature of the resulting product depends on the counterion.