The synthesis and characterization of the novel zirconium (IV) tris(pyrazolyl)borate compound {Tp(Ms)*}ZrCl3 (1) (Tp(Ms)* = hydridobis(3-mesitylpyrazol-1-yl)(5-mesitylpyrazol-1-yl)), as well as its performance in polymerizing ethylene are described. The reaction of ZrCl4 with 1 equivalent of TlTp(Ms)* in toluene at room temperature affords 1 as a white solid in 62% yield. Compound 1 in the presence of MAO showed remarkable productivity using a low Al:Zr molar ratio (6.79 x 10(4) kg of PE/(mol Zr . h . [C2H4]); toluene, 60 degreesC, Al/Zr = 100). Under identical polymerization conditions, compound 1 and Cp2ZrCl2 showed comparable productivities. Compound 1 displayed similar productivities at temperatures in the range of 0-75 degreesC and noticeable productivity at 105 degreesC. The viscosity-average molecular weight of the polyethylenes depends on the Al:Zr molar ratio and polymerization temperature and varied between 1.09 and 8.98 x 10(5) g . mol(-1).