Kinetic measurements were undertaken to better understand the hydrometallation step. Eyring analysis was conducted over a temperature range 70 -100 °C. Kinetic data for the addition of 1 to 3c was modelled as first order in 1. The activation parameters for hydromagnesiation were found to be: 11.2 ± 2.9 cal K -1 mol -1 . A related reaction of cis-[Rh(P i Pr3)2(H)( 2 -CH2=CH2)] proceeds with ΔH ‡ = 13.0 kcal mol -1 and ΔS ‡ = -2.0 cal K -1 mol -1 . Although both processes occur with similar activation enthalpies, the entropy of activation of hydromagnesiation is large and negative, suggestive of a substantial increase in order at the transition state required for intermolecular hydromagnesiation compared to intramolecular alkene insertion at transition metals. We have previously reported ring-opening of methylidene cyclopropane (3a) with 1 to form 6a. Hence, with these Mgreagents C-C bond activation of 3-and 4-membered hydrocarbon rings is possible, but not larger 5 or 6-membered cycloalkanes.