The niobium and tantalum imido complexes [CpMCl 2 (NDipp)], [MCl 3 (NR)(dme)] (R = t Bu, Ph, 2,6-i Pr 2 C 6 H 3 (Dipp), and Mes), and [TaCl 3 (NDipp)(tmeda)] were tested in combination with EtAlCl 2 for the dimerization of ethylene. The niobium systems afforded dimers or polymers, depending on the nature of the imido ligand, with overall productivities in the range 720 to 13,720 (mol C 2 H 4 )(mol Nb) À 1 . The nature of the polyethylene produced (LDPE or HDPE) depended on the imido ligand and the niobium concentration at which catalysis was run. In contrast, the tantalum/dme systems all mediated ethylene dimerization with productivities of up to 4,503 (mol C 2 H 4 )(mol Ta) À 1 , with overall selectivities to butenes of between 73-81 wt %; selectivity within the dimer fraction to 1-butene was in the range 72 to 100 %. The productivity of [TaCl 3 (NDipp)(tmeda)] was six times higher than that of its dme-bearing counterpart, but at the cost of selectivity to 1-butene. For the tantalum imido-mediated ethylene dimerization the composition of the product slate formed is indicative of a metallacyclic mechanism being operative.[a] Dr.