Manganese(II) halogeno complexes containing a neutral tris(3,5-diisopropyl-1-pyrazolyl)methane (referred to as L1′) ligand have been examined on their catalytic performance in ethylene polymerization and ethylene/1-hexene copolymerization. The activities of [Mn(Cl 2 )(L1′)] (a) and [Mn(Br)(L1′)](Br) (b) activated by Al(i-Bu) 3 /(Ph 3 C)[B(C 6 F 5 ) 4 ] for ethylene polymerization go up to 1600 and 840kgmol(cat) −1 h −1 with a bimodal molecular weight distribution (M w /M n ) of 4.1 and 4.2, respectively. These results are different from the corresponding manganese(II) complexes with hydrotris(pyrazolyl)borate anion.