The Schiff base N,N‘-o-phenylenebis(3,5-di-tert-butyl-salicylideneimine) (1; tBu4salophenH2) reacts with 1 equiv of Zr(η5-C5H5)Cl3·DME in the presence of 2.2 equiv of NEt3 in pentane to give the monocyclopentadienyl bis(phenoxo-imino) monochloro zirconium derivative Zr(η5-C5H5)[C6H4-1,2-{NCH(3,5-tBu2C6H2-2-O)}2]Cl (2). Moreover, 2 equiv of the monolithiated reagent derived from 3-tert-butyl-N-ethylsalicylaldimine by reaction with LinBu in hexane reacts at −78 °C with 1 equiv of Zr(η5-C5H5)Cl3·DME in THF to yield the corresponding monocyclopentadienyl bis(phenoxo-imino) monochloro zirconium complex Zr(η5-C5H5)[CH3CH2NCH{(3-tBuC6H3-2-O)}]2Cl (5) after elimination of LiCl. Methylation of 2 with MgClMe in pentane or toluene at −78 °C affords Zr(η5-C5H5)[1,2-C6H4{NCH(3,5-tBu2C6H2-2-O)}{NCH(Me)(3,5-tBu2C6H2-2-O)}] (3) as a result of the reduction of one of the CN bonds of the coligand. The hydrolysis of 3 gives the new organic compound [C6H4{NCH(3,5-tBu2C6H2-2-OH)}{NCH(Me)(3,5-tBu2C6H2-2-OH)}] (4), which was also quantitatively isolated by reaction of 1 with MgClMe followed by treatment with a saturated solution of NH4Cl in water. All of the reported compounds were characterized by the usual analytical and spectroscopic methods, and the molecular structure of 5 was determined by X-ray diffraction analysis from suitable single crystals. The catalytic activities of 2, 3, and 5 for ethylene polymerization using MAO as cocatalyst were determined.