Manganese(II) complex catalysts with hydrotris(pyrazolyl)borate ligands have been examined on their catalytic performance in ethylene polymerization and ethylene/1-hexene copolymerization. The activities of [Mn(L6)(Cl)(NCMe)] (1) and [Mn(L10)(Cl)] (2) activated by Al(i-Bu)(3)/[Ph3C][B(C6F5)(4)] for ethylene polymerization go up to 326 and 11 kg mol (cat(-1)) h(-1), respectively, (L6(-) = hydrotris(3-phenyl-5-methyl-1-pyrazolyl)borate anion, L10(-) = hydrotris(3-adamantyl-5-isopropyl-1-pyrazolyl)borate anion). In particular, for ethylene/1-hexene copolymerization, complex 1 gives high-molecular-weight poly(ethylene-co-1-hexene)s with the highest M-w, of 439,000 in manganese olefin polymerization catalyst systems. Moreover, the 1-hexene incorporation by complex 1 seems more efficient than that by [Mn(L3)(Cl)] (4) (L3(-) = hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate anion). In this work, we demonstrated that the coordination geometry and coordination number are also important factors for ethylene polymerization reaction as well as steric hindrances and ligand frameworks in our manganese(II) catalysts. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5720-5727, 2009