In 2020, we reported C-C -bond cleavage of strained alkylidene cyclopropanes using magnesium reagents (Scheme 3, top). This work was extended to include reaction of the related magnesium(II) hydride complex 6 [Mg{(-H){CH{C(CH3)NDipp}2}]2 (Dipp = 2,6-diisopropylphenyl) with the same set of substrates (Scheme 3, bottom). Stoichiometric reaction of 6 with methylidene cyclopropane (7a) and methylidene cyclobutane (7b) yielded the ring-opened alkenyl magnesium complexes 9a and 9b in good yields. Though no intermediates were observed spectroscopically, DFT calculations and related literature supported a hydromagensiated intermediate 8a-b as a prerequisite to -alkyl elimination and thus C-C bond activation. Additional evidence for the hydromagensiated intermediates was gathered through reaction of unstrained methylidene cyclopentane (7c) and methylidene cyclohexane (7d) with 6, which formed the hydromagnesiated products 8c-d in high yields. Calculated activation barriers for -bond C-C bond cleavage at 8c-d were unfeasible under the reaction conditions (G ‡ 298K > 40 kcal mol -1 ), indicating that the release of ring strain in the three-and four-membered systems is an important driving force for the reaction. Activation strain analysis was used to explain the differences in reactivity between the analogous zinc and magnesium hydride complexes. Namely, that magnesium was observed to ring open cyclobutane rings, whereas zinc was not. The more electropositive metal (Mg) was shown to be better able to stabilize the hydrocarbon fragment at the C-C activation transition state, lowering the kinetic barrier relative to Zn. 6 and 10 have an identical ligand coordination, at such it can be concluded that chemoselectivity in these reactions can be controlled through choice of the main-group metal.