The unique electronic properties of the strained bicyclo-[1.1.0]butanes [1] are enhanced by the heteroatoms in the molecular frame. Illustrative is the bond-stretch isomerization of the P 2 C 2 and P 2 B 2 bicycles.[2] However, very few systems are known with a single heteroatom, [3] probably because of their high reactivity, which is only moderated when the heteroatom occupies a bridgehead position as in the 1-aza derivatives. [4] The increased reactivity of the hetero systems is due to the valence isomerization to which the bicyclo[1.1.0]butanes are prone. We now report on the first 2-phospha derivatives.The carbene-like phosphinidene Ph-P = W(CO) 5 [5] was generated in situ by cheletropic elimination from 1 at 110 8C in toluene and then allowed to react with cyclopropene 2 a [6] (Scheme 1). This led to the desired W(CO) 5 -complexed 2-phosphabicyclo[1.1.0]butanes exo-3 a (d 31 P = À85.1 ppm) and endo-3 a (d 31 P = À36.7 ppm) [7] in a 10:9 ratio, which were isolated in 69 % yield as colorless solids. The remarkably