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To gain a deeper knowledge of the nature of the species formed in situ by reaction of a catalyst precursor complex and typical cocatalysts used for ethylene oligomerization or polymerization, the Cr(III) complex fac-[Cr(NPN)Cl3] (1; NPN = bis(2-picolyl)phenylphosphine) was reacted with MAO and partial methylation to give the mononuclear complex fac-[Cr(NPN)Cl2.23Me0.77] (2) was observed, whereas reaction with AlMe3in toluene led to partial halide abstraction and produced the dinuclear, chloride-bridged dicationic complex [{fac-Cr(NPN)Me(μ-Cl)}2][AlMexCl4–x]2(3·[AlMexCl4–x]2), in which the metal centers are bridged by two chloride ligands. The complex fac-[Cr(NPN)Cl2Et] (4) was isolated in high yield from the reaction of 1with AlEt3and contains a rare example of CrIII–Et function. Upon treatment of 1with EtAlCl2, the dinuclear Cr(III) complex [{fac-Cr(NPN)}2(μ-Cl)3][AlCl4]3(5·[AlCl4]3) was obtained, which contains three bridging chlorides. Thus, under the conditions investigated, MAO and AlEt3behaved as alkylation agents, whereas AlMe3and EtAlCl2reacted as cationization agents. On activation with MAO and under comparable conditions, 1–5exhibited similar catalytic properties, with activities between 2700 and 2800 g of C2H4/((g of Cr) h), but the major oligomer produced using the 4/MAO system was 1-decene. The neutral alkyl complexes 2and 4and the cationic complexes 3and 5did not exhibit catalytic activity for ethylene oligomerization in the absence of MAO. The structures of the complexes 2, 3·[AlMexCl4–x]2, 4, and 5·[AlCl4]3have been determined by single-crystal X-ray diffraction. [ABSTRACT FROM AUTHOR]