Lithiation of the β-diketimines (2,6-C6H3R2)NH═C(Me)CH═C(Me)N(2,6-C6H3R2) (R = Me (HL1), Et (HL2)) by nBuLi was followed by metathesis reaction with LnCl3(THF)x and Y(BH4)3(THF)2 to afford the corresponding complexes L1LnCl2(THF)2 (Ln = Gd (1), Nd (3), Dy (4), Er (5), Y (6)), L2GdCl2(THF)2 (2), and L1Y(BH4)2(THF) (8), respectively. Treatment of neutral HL1 with Y(CH2SiMe3)3(THF)2 generated the bis(alkyl) complex 7, L1Y(CH2SiMe3)2(THF). Upon activation with [PhNHMe2][B(C6F5)4] and AliBu3, complex 6 showed the highest cis-1,4 selectivity (99.3%, Tp = 0 °C) toward the polymerization of isoprene, while complex 7 had a comparatively low cis-1,4 selectivity, and in contrast, complex 8 was completely inert. The influences of the ortho substituents of the N-aryl rings of the ligands, the types of central metals and cocatalysts, and addition sequence of the catalyst components had been thoroughly investigated. By means of X-ray diffraction and 1H NMR spectroscopy analyses, the intermediates arising from the stoichiometric reactions among the catalyst components and the probable active species were elucidated, which facilitates further investigation of the mechanism for diene polymerization.