Hydrometallation of methylidene cycloalkanes with either 1 or 2 both proceed via a four membered transition state TS-1 (Figure , Table ). No clear trend between ring size and activation barrier was noted. The nature of the main-group metal was determined to be the most important factor in determining the activation barrier for hydrometallation. Gibbs activation energies for the hydrometallation transition state TS-1 ranged from 21.7 -24.0 kcal mol -1 for Mg, and 30.7 -32.5 kcal mol -1 for Zn (Table ). These calculations are consistent with higher temperatures and longer reaction times observed experimentally upon reaction of 2 with methylidene cycloalkanes compared to 1. In all cases, the hydrometallation step is exergonic with the most energetically stable products being those in which there is relief of strain due to sp 2 to sp 3 rehybridisation on hydrometallation. For example, the difference in strain energy between methylidene cyclopropene and cyclopropane can be estimated as 13.4 kcal mol -1 . 37 Natural Bond Orbital (NBO) analysis of the calculated stationary points for the hydrometallation step were undertaken (Table ). Analysis of the Wiberg Bond Index (WBI) on stationary points across the pathway shows the expected decrease in C=C bond strength that occurs with loss of the unsaturated bond in 3a-b upon addition of 1 or 2. Take addition of 1 to 3b as a representative example;