Adducts of Carbenes with Group II and XII Metallocenes^†

The reactions and adducts of 1,3,4,5-tetramethylimidazol-2-ylidene with bis(pentamethylcyclopentadienyl)magnesium, -calcium, -strontium, -barium, or -zinc are described. A consistent tend in the spectroscopic properties and structures of the alkaline earth metallocenes is observed. The adduct between 1,3,4,5-tetramethylimidazol-2-ylidene and bis(pentamethylcyclopentadienyl)zinc shows a structure in which the zinc is only σ-bonded to the three ligating groups. Among the alkaline earth adducts, the σ-bonded interaction to the former carbene center of the 1,3,4,5-tetramethylimidazol-2-ylidene shows the shortest metal−carbon distances in each of the structures but the difference between the carbene−metal and pentamethylcyclopentadienyl−metal distances decreases as the atomic number of the metal increases.