The highly sterically encumbered chelating beta-diketiminate ligand, [HC{C(Me)N(2,6-CHPh2-4-MeC6H2)}(2)](-), L-Ar(-), has been used to prepare a series of heteroleptic three-coordinate magnesium complexes. Both the bis(imine) and imine-enamine tautomers of the ligand precursor, (LH)-L-Ar, as well as the diethyl ether adduct of the bromide complex [(LMgBr)-L-Ar(OEt2)], the monomeric methyl complex [(LMgMe)-L-Ar], the THF-solvated and unsolvated n-butylmagnesium complexes [(LMgBu)-L-Ar-Bu-n(THF)] and [(LMgBu)-L-Ar-Bu-n], and the 1-hexynyl analogue [(LMgC)-L-Ar (CBu)-Bu-n] have been crystallographically characterized. Both n-butylmagnesium complexes showed remarkable stability in air, both in the solid state and in solution. Single crystals of the highly sensitive magnesium hydride, [(LMgH)-L-Ar], underwent partial hydrolysis by solid-state water diffusion to the isostructural hydroxide compound [(LMgOH)-L-Ar].