In summary, C-C -bond activation of strained cyclic hydrocarbons has been observed to occur at both Mg and Zn metal centres. While ring-opening of both 3-and 4-membered rings was observed with Mg reagents, isostructural Zn species were only capable of reacting with smaller 3-membered rings. DFT calculations, benchmarked against the experiment data and activation parameters from kinetics, support a mechanism for C-C -bond activation involving -alkyl migration. While relief of ring strain provides a thermodynamic driving force for -alkyl migration, activation strain analysis suggests that it does not lower the barrier for breaking the C-C -bond by an appreciable amount.