The dinuclear [NbCln(OR)((5-n))](2) (n = 4, R = Et, 1; n = 4, R -CH2Ph, 2; n - 3, R - Et, 3; n - 2, R - Et, 4; n - 2, R -CH(CH2)(4)CH(Cl), 5), and [Nb(OEt)(5)](2), 6, and the mononuclear niobium compounds NbCl4[kappa(2)-OCH2CH(R')OR] (R = Me, R' = H, 7; R = Et, R' = H, 8; R = CH2Cl, R' = H, 9; R = CH2CH2OMe, R' = H, 10; R = R' = Me, 11), NbBr4[kappa(2)-OCH2CH2OMe], 12, and NbCl3(kappa(2)-OCH2CH2OMe)(kappa(1)-OCH2CH2OMe), 13, were tested in ethylene polymerization. Optimized reaction conditions included the use of D-MAO as co-catalyst and chlorobenzene as solvent at 50 degrees C. Complex 7, whose X-Ray structure is described here for the first time, exhibited the highest activity ever reported for a niobium catalyst in alkene polymerization [151 kg(polymer) x mol(Nb)(-1) x h(-1) x bar(-1)]. Compounds 1, 3-5, 8, 13 showed activities similar to that of 7. Linear polyethylenes (characterized by FT-IR, NMR, GPC, and DSC analyses) with a broad polydispersivity were obtained. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 1664-1670, 2011