Starting from bis(allyl)magnesium [Mg-(C3H5)(2)], a set of cationic, neutral, anionic, and dianionic allyl magnesium compounds has been isolated and characterized, including [Mg(C3H5)(THF)(5)][B(C6F5)(4)] (3), [Mg-(C3H5)(2)(1,4-dioxane)(THF)] (2), [KMg(C3H5)(3)(THF)] (6), and [MMg(C3H5)(4)] (8: M = K-2; 9: M = Ca). In solution, the allyl ligands of the compounds display fluxional behavior, even at low temperatures. Single crystal X-ray analysis reveals unusual mu(2)-eta(1):eta(3)- and unprecedented mu(3)-eta(1):eta(3):eta(3)-coordination modes in the heterobimetallic compounds 6 and [8.(THF)(2)]. Density functional theory calculations confirm that these metal-allyl conformations are energetically stable. The magnesium compounds have been investigated as initiators for butadiene polymerization and ethylene oligomerization. The heterobimetallic compounds display initiation properties, including higher reaction rates, that are distinctively different from those of the monometallic species. Reactivity trends depend on the formal charge of the magnesium compounds (dianionic, higher-order magnesiate > monoanionic, lower-order magnesiate) and on the nature of the counterion (K+ > Ca2+).