An ionic chromium(III) species, [CrCl2(THF)(4)][Al(OC4F9)(4)] (2), has been synthesized and is shown to react with a variety of bidentate diphosphine ligands to yield complexes of the type [CrCl2(diphosphine)(2)][Al(OC4F9)(4)]. When compound 2 is combined with diphosphines Ar2PN(Me)PAr2 (Ar = 2-MeO-C6H4) and Ph2PN(i-Pr)PPh2, after activation with small amounts of AlMe3, active species for the selective oligomerization of ethylene with catalyst productivities of up to 25010 g g(Cr)(-1) h(-1) are observed. Selectivity to 1-hexene or 1-octene is a function of ligand structure in an identical fashion to the MAO-activated system. A novel Cr(II) species, [Cr(Ar2PN(Me)PAr2)(2)][Al(OC4F9)(4)](2) (5), was isolated from the catalytic mixture. Compounds alone does not oligomerize ethylene, but reaction with MAO yields a highly active catalyst for selective ethylene oligomerization.