Abstract: The synthesis of diazotetrakis (trifluoromethy1) cyclopentadiene (1) is described. This stable perfluorinated diazo compound undergoes a photochemical loss of nitrogen to yield a reactive intermediate which reacts with a variety of elements bearing lone pairs to give tetrakis (trifluoromethy1) cyclopentadiene ylides. The synthesis of carbonyl, thiocarbonyl, and halonium ylides is described as well as evidence for oxonium and ...