The highly sterically encumbered chelating β-diketiminate ligand, [HC{C(Me)N(2,6-CHPh2-4-MeC6H2)}2](-), (Ar)L(-), 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, (Ar)LH, as well as the diethyl ether adduct of the bromide complex [(Ar)LMgBr(OEt2)], the monomeric methyl complex [(Ar)LMgMe], the THF-solvated and unsolvated n-butylmagnesium complexes [(Ar)LMg(n)Bu(THF)] and [(Ar)LMg(n)Bu], and the 1-hexynyl analogue [(Ar)LMgC≡C(n)Bu] 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, [(Ar)LMgH], underwent partial hydrolysis by solid-state water diffusion to the isostructural hydroxide compound [(Ar)LMgOH].