Activation Strain Analysis on the potential energy surface associated with -alkyl migration transition state TS-2 suggests that there is only a slight difference in the contribution from strain energy in these two reactions (Figure ). For both systems, the reaction strain energy Estrain() reaches a maximum at the transition state. For Mg, there is little difference in the strain energy, E ‡ strain from reactant to transition state, TS-2 (n =1, 12.0 kcal mol -1 ; n =2, 11.9 kcal mol ). Nearly all the contribution to the strain energy is from distortion of the hydrocarbon fragment. On further inspection of the transition state geometries this conclusion becomes more intuitive, there are only small distortions in bond angles in the ring systems of these structures when compared to the parent cycloalkanes. Hence, relief of ring strain does not occur at the transition state but rather later along the reaction coordinate as the system progresses toward the products. The interaction energy Eint() is negative and relatively flat across the reaction coordinate. This is perhaps expected, as the calculated data points describe a concerted bond breaking and bond making event in which stabilising interactions from forming bonds are counterbalanced by destabilising interactions from breaking bonds. From reactant to transition state, the change in interaction energy is positive, suggestive of a less stabilising interaction between the metal site and hydrocarbon fragment as the TS is approached. For the Mg system, comparing E ‡ int for TS-2 (n =1, 1.1 kcal mol -1 ; n =2, 7.7 kcal mol -1 ) suggests that the origin of the higher activation energy for 4-membered vs 3-membered rings can be traced to a weaker binding of the hydrocarbon to the metal. Based on these calculations, it can be speculated that more facile activation of the smaller ring sizes is not a consequence of relief of ring strain in the transition state, but rather a more stabilising interaction of the allyl type ligand with the metal at the TS.