A Homologous Series of Base‐Free Organo(alkaline‐earth) Metallocenes: Synthesis and Molecular Structures of [1,2,4‐(SiMe 3 ) 3 C 5 H 2 ] 2 (Ca, Sr, Ba)

Bis[1,2,4‐tris(trimethylsilyl)cyclopentadienyl] complexes [1,2,4‐(SiMe 3 ) 3 C 5 H 2 ] 2 Ae [(Cp 3Si ) 2 Ae; Ae = Ca ( 1 ), Sr ( 2 ), Ba ( 3 )] are isolated from the 2:1 reaction of K[Cp 3Si ] and AeI 2 in diethyl ether. Under the same reaction conditions used for the heavier metallocenes, the corresponding (Cp 3Si ) 2 Be compound was not formed, although calculations suggest that the beryllocene would be sterically feasible. Compound 1 crystallizes as a bent monomer from hexanes (ring centroid−Ca−ring centroid = 166.7°) with two η 5 ‐Cp 3Si ligands on the calcium atom [Ca−C(av) = 2.63(2) Å]. Metallocene 2 , the first crystallographically characterized example of an unsolvated strontocene, is also isolated as a bent monomer (159.4°) from hexanes with two η 5 ‐Cp 3Si ligands on the strontium atom [Sr−C(av) = 2.819(2) Å]. Compound 3 crystallizes on sublimation as a coordination dimer in which each barium atom is flanked by two η 5 ‐Cp 3Si rings [Ba−C(av) = 3.01(2) Å] and one methyl group from a neighboring Cp′ ring; the intermolecular Ba ··· C(methyl)′ distance is 3.275(6) Å. The amount of bending observed in 1 − 3 and other Group‐2 metallocenes varies with the metals, the steric bulk of the cyclopentadienyl ligands, and crystal packing effects, and is not easily predictable. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)