The ligand exchange reaction of a diene ligand bound to magnesium was investigated. Reaction of the magnesium-butadiene compound [Mg(C 4 H 6 )(thf) 2 ] n (2) with 1,4-diphenyl-1,3-butadiene afforded Mg(s-cis-1,4-diphenyl-1,3-butadiene)(thf) 3 (1) together with butadiene. Similarly, treatment of 2 with 1,6-diphenyl-1,3,5-hexatriene, anthracene, and 1,3,5,7-cycIooctatetraene afforded the corresponding magnesium adducts Mg(1,6-diphenyl-1,3,5-hexatriene)(thf) 3 (3) Mg(anthracene)(thf) 3 (4), and Mg(cot)(thf) 2,5 (5), respectively, in addition to the liberated butadiene. Reaction of 2 with diphenylacetylene in THF also induced the ligand exchange reaction, resulting in the formation of a diphenylacetylene adduct [Mg(PhC 2 Ph)(thf)] 4 (6) of magnesium.