Reaction of 1a with cyclopropene 2a, at 25 °C in C6D6 led to the rapid formation of the 1,2-dimagnesiocyclopropyl complex 3a. Similar reactivity was observed between 1a and 2b (15 min at 25 °C) or 2c (4h at 60 °C), leading to the corresponding species 3b and 3c. The slowest reaction occurs with the most sterically hindered substrate. In all cases, the selective syn-insertion of the alkene into the Mg-Mg bond was observed. For 2a and 2b, attack of 1a occurs on the least hindered face of the ring system leading to the formation of a single diastereomer of the product (Scheme 1). The Mg-C bonds of 2.02(1) and 2.20(1) Å are in the normal range for magnesium alkyls. Additional support for the proposed structure being retained in solution was obtained through in situ generation of an asymmetric analogue of 3a (with different ligands on each Mg centre). In this case, the two protons of the cyclopropane moiety become inequivalent. Analysis of the 1 H NMR spectrum of the crude reaction mixture indicates a mutual 3 JHH=13.6 Hz coupling for the cyclopropane protons which is consistent with known literature values for syn-stereochemistry. Addition of 2a-c (2 equiv.) to 1b, a less sterically hindered analogue of 1a, also proceeded extremely rapidly at 25 °C in C6D6 solution. In these cases, however, a double insertion reaction occurred, forming 4a-c (Scheme 2). This reaction likely proceeds by two sequential steps: (i) a syn-selective insertion of the cyclopropene into the Mg-Mg bond of 1b and (ii) a syn-selective carbomagnesiation of a second equivalent of cyclopropene from the intermediate 1,2-dimagnesiocyclopropyl complex. This type of reactivity has already been reported by Jones and co-workers for reaction of 1b with ethylene in the presence of an N-heterocyclic carbene ligand. In the case of 2c, an intermediate 3d could be spectroscopically observed. Addition of a further equiv. of cyclopropane to this intermediate led t o 4c.