A room-temperature-stable crystalline 2H-phosphirene (1) was prepared by treatment of an electrophilic diamidocarbene with tert-butylphosphaalkyne. Compound 1 is shown to react as a vinylphosphinidene generated via phosphirene-phosphinidene rearrangement. Thermolysis is shown to affect C-N bond scission while reactions with C6Cl4O2 or (tht)AuCl afford formal oxidation of the phosphindene center and the phosphinidene-insertion into an aromatic C-C bond of a mesityl group, respectively. The latter reaction is the first example of a phosphorus analog of the Büchner ring expansion reaction.