The reaction of anhydrous YCl3 with an equimolar amount of lithium N,N'-diisopropyl-N'-bis(trimethylsilyl)guanidinate, Li[(Me3Si)(2)NC(Ni-Pr)(2)], in tetrahydrofuran (THF) afforded the monomeric monoguanidinate dichloro complex {(Me-3-Si)(2)NC(Ni-Pr)(2)]YCl2(THF)(2) (1). Alkylation of complex 1 with 2 equiv of LiCH2SiMe3 in hexane at 0 degrees C yielded the monomeric salt-free dialkyl complex {(Me3Si)(2)NC(Ni-Pr)(2)]Y(CH2SiMe3)(2)(THF)(2) (2). The bis(triethylborohydride) complex [(Me3Si)(2)NC(Ni-Pr)(2)]Y[(mu-H)(mu-Et)(2)BEt](2)(THF) (5) was prepared by the reaction of complex 1 with 2 equiv of LiBEt3H in a toluene-THF mixture at 0 degrees C. The complexes 1, 2, and 5 were structurally characterized. Complex 2 as well as the systems 2-Ph3B, 2-Ph3B-MAO, and 1-MAO. (MAO = methylaluminoxanes) in toluene were inactive in ethylene polymerization, while the product obtained in situ from the reaction of complex 2 with a 2-fold molar excess of PhSiH3 in toluene polymerized ethylene with moderate activity.