Magnesium allyl complexes are regularly isolated with classical, sigma-bonded ligands, and this has been thought to be their preferred mode of bonding. Density functional theory calculations confirm that such bonding is the most stable mode when coordinated bases are present, but in their absence, pi-bonded forms are expected to be lower in energy. The isolation of the unsolvated [Mg{C(3)(SiMe(3))(2)H(3)}(2)](2) complex supports this prediction, as it is a dinuclear species in which two allyl ligands bridge the metals and display cation-pi interactions with them.