In summary, we have developed an experimental and computational understanding of the stereochemistry of 1,2dimagnesiation of cyclopropenes. Key to this understanding is the realisation that the reaction occurs by a concerted syn-addition transition state in which the Mg-Mg bond adds across the most sterically accessible face of the alkene. These findings may have broad implications for the design of stereoselectivity reactions involving the addition of magnesium reagents to unsaturated carbon-carbon and carbon-heteroatom bonds.