(η5-Cyclopentadienyl)(η4-tetrasila- and η4-trisilagermacyclobutadiene)cobalt:: Sandwich complexes featuring heavy cyclobutadiene ligands

( eta(5)-Cyclopentadienyl) (eta(4) -tetrasilacyclobutadiene) cobalt {[(eta(4)-R4Si4)COCP], 2} and (eta(5)-cyclopentadienyl)(eta(4)-trisilagermacyclobutadiene)cobalt {[(eta(4) -R4Si3Ge)CoCpl, 4} (R = Si MetBu(2)) were synthesized by reaction of the dipotassium salts of tetrasilacyclobutadiene dianion K(+)2 center dot[R4Si4](2-) (1) and trisilagermacyclobutadiene dianion K(+)2 center dot[R4Si3Ge](2-) (3) with [CPCOI2(PPh3)]. Alternatively, 4 was prepared by the reaction of 3 with [CP2CO](+)center dot[PF6](-). X-ray crystallographic analysis of 2 confirmed its sandwich-type structure, manifesting a nearly square-planar Si-4 ring and diagnostic perhaptocoordination of both ligands, eta(4)-tetrasilacyclobutadiene and eta(5)-cyclopentadienyl, to the Co atom. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).