i) hydromagnesiation of the alkene through a 1,2-insertion reaction of the magnesium hydride to the alkene ii) -alkyl migration; iii) -bond metathesis to regenerate magnesium hydride catalyst 6 and the linear silane. The thermodynamic products 13-16 are likely formed from an intramolecular hydrosilylation also catalysed by the magnesium complex 6. The related zinc complex 10 was unable to catalyse the hydrosilylation of the -C-C bond of 7a, again highlighting the divergent reactivity of different main-group metals.