Polarisation and charge delocalisation across the C=C---Cy moiety may also play a role in the differences in reactivity for Mg and Zn. Transition states for C-C bond activation are calculated to be higher in energy for Zn than for Mg across the whole series. For Zn, activation strain analysis again suggests that there is little difference in the relief of ring strain on going from reactant to the transition state E ‡ strain for TS-2 (n =1, 11.1 kcal mol -1 ; n = 2, 12.9 kcal mol -1 ). Rather the interaction energy is again the key factor at play. E ‡ Int for TS-2 (n = 1, 8.1 kcal mol -1 ; n = 2, 19.7 kcal mol -1 ) is less destablising for the smaller ring size. In addition, E ‡ Int values for TS-2 for both ring sizes are less destabilising for Mg compared to Zn. The Mg site is both larger and more electropositive than Zn (rcov Mg = 1.39, Zn = 1.18 Å; p Mg = 1.31, Zn = 1.65). As such it is likely better placed to polarise the C=C---Cy moiety and stabilise the resultant fragment through electrostatic interactions with all three carbon centres. The Zn atom is less well suited to accommodate coordination of all three carbon atoms involved in C-C bond activation. This supposition is supported by the calculated TS geometries for Zn which show a greater asymmetry in the Zn-C and Zn-C distances compared to Mg analogues. For example, in TS-2 (n = 1) there is a  = 0.22 Å difference in these values for Zn, whereas it is only  = 0.05 Å for Mg. We speculate these differences make an important contribution to raising the energy of the C-C bond activation step for Zn relative to Mg.