Magnesium bromide-tetrahydrofuran complexes: bis(tetrahydrofuran)magnesium bromide, tris(tetrahydrofuran)magnesium bromide, tetrakis(tetrahydrofuran)magnesium bromide, and diaquotetrakis(tetrahydrofuran)magnesium bromide. A reagent for the preparation of anhydrous magnesium phosphodiester salts