Deprotonation of sodium acetylcyclopentadienide (11) was achieved by treatment with LDA in THF to generate the dianion equivalent [Cp-C(=CH2)-O](2-) (12). Transmetalation with Cl2Ti(NMe2)(2) gave {[Cp-C(=CH2)-O]Ti(NMe2)(2)}(2) (17); treatment of 12 with Cl2Zr(NEt2)(2)(THF)(2) furnished {[Cp-C(=CH2)-O]-Zr(NEt2)(2)}(2) (18). Cryoscopy in benzene revealed a dimeric structure of 18 in solution. Complex 18 was characterized further by an X-ray crystal structure analysis and by DFT calculations. The two zirconium centers of 18 are connected by means of two symmetry-equivalent eta(5):kappaO[Cp-C(=CH2)-O] ligands. The ligand backbone shows no specific steric constraints, different from the formally related "constrained geometry" systems such as [Cp*-SiMe2-NCMe3]Zr(NMe2)(2) (1 b). Nevertheless, upon treatment with MAO the CpCO group 4 metal complex system (18) generates an active homogeneous Ziegler-Natta catalyst for effective ethene/1-octene copolymerization, with up to 20% 1-octene having become incorporated in the resulting copolymer at 90 degreesC.