Diphosphadiazanediyl ([P(mu-NTer)](2) 1) is known to readily react with small molecules bearing multiple bonds to give [2.1.1]bicyclic species. On the contrary, in the reaction of isonitriles with 1, planar five-membered heterocycles (3) with biradical character are formed by insertion of the carbon atom into one P-N bond. Under irradiation, heterocyclic biradicaloids 3 are shown to generate housane-type [2.1.0]-bicyclopentanes by transannular bond formation. However, these housane species thermally equilibrate, reforming the open-shell singlet cyclopentanediyl. The biradical character of 3 indicates high reactivity which is further demonstrated in the activation of small molecules bearing multiple bonds leading to [2.2.1]bicyclic heterocycles. Depending on the substituent of the isonitrile, the reaction with a second equivalent of isonitrile is also observed for smaller substituents. By employing suitable diisonitriles, even the catenation of two open-shell singlet cyclopentane-1,3-diyls is achieved. CASSCF(6,6) computations revealed biradical character for 3 ranging between 26 and 27%.