Polymerization of ethylene catalyzed by half-metallocene complexes of niobium and tantalum MR 2 (η 5 -C 5 Me 5 )(L) (1: M = Ta, R = Cl, L = o-xylylene; 2: M = Nb, R = Cl, L = o-xylylene; 3: M = Ta, R = Cl, L = η-1-4-anthracene; 4: M = Ta, R = CH 2 Ph, L = η-1-4-anthracene) as well as by a new anthracene-zirconium complex [Mg 2 Cl 3 (THF) 6 ][ZrCl 2 (η 5 -C 5 Me 5 )(η-1-4-anthracene)] (5), whose structure was revealed by X-ray analysis, was studied and the results were compared with the reported results using 1,3-butadiene complexes of niobium and tantalum. The catalytic activity of niobium and tantalum complexes was found to be in the order of o-xylylene > butadiene anthracene, whereas the anthracene complex of zirconium 5 showed much lower activity and broad polydispersity.