Beyond their archetypal Grignard-type reactivity [1] organoA C H T U N G T R E N N U N G metallic complexes of the heavier alkaline-earth metals are currently attracting a great deal of attention in homogeneous catalysis. [2, 3] The large ionic radii of divalent M II (M = Ca, Sr, Ba), the highly ionic nature of the respective metal-carbon s bonds, and the associated huge ligandexchange rates [4] imply high reactivity in catalytic transformations such as olefin polymerization, hydrosilylation, or hydroamination. Naturally, these bonding and reactivity attributes are most pronounced for Ba II and are held responsible for the lack of well-defined and crystallographically authenticated alkyl derivatives. In 1998, Westerhausen and coworkers presented the X-ray structure of the dimeric com-
plex [((thf)Ba{CA C H T U N G T R E N N U N G (C CPh)(Ar)}) 2 ] with Ba À CA C H T U N G T R E N N U N G (alkenyl) bonds, which was obtained by the reaction of [(thf)Ba{PA C H T U N G T R E N N U N G (SiMe 3 ) 2 } 2 ]with diphenylbutadiyne.[5] Later on, the same group described the reaction of, which consists of a Ba 2 C 2 P 4 Zn 4 cuboid with Ba-Et-Zn moieties.[6] The majority of well-characterized dialkyl barium compounds, however, feature a-bound aryl and trialkylsilyl groups for stabilization. Representative examples include [Ba-