The salen anion forms complexes with most transition metals. These complexes are usually prepared by the reaction of Hsalen ("proligand") with metal precursors containing built-in bases, such as alkoxides, metal amides, or metal acetate. The proligand may also be treated with a metal halide, with or without an added base. Lastly, the proligand may be deprotonated by a nonnucleophilic base, such as sodium hydride, before treatment with the metal halide. For example, Jacobsen's catalyst is prepared from the salen ligand precursor with manganese acetate.