Brookhart's nickel α-diimine complex [(κ₂-N,N-BIAN)NiCl₂] (1) (where BIAN = {Ar–NAceN–Ar}, Ace = acenaphthen-1,2-diyl, and Ar = 2,6-(iPr)₂-C₆H₃) activated with a hydrosilane/B(C₆F₅)₃ (SiHB) adduct forms a highly active catalytic system for ethylene polymerization. Under optimal conditions, the activity of the system depends on the nature of hydrosilane and decreases in the order R₃SiH > Ph₂SiH₂ > PhSiH₃. The decrease in system activity within the hydrosilane series is correlated with increasing formation of Ni(I) species. In addition to their activation effect, hydrosilanes act as efficient chain termination/chain transfer agents, with the Si/Ni ratio controlling the molecular weight of the resulting polyethylene (PE). The use of Et₃SiH generated elastomeric, highly branched polymers with a saturated chain-end, while systems using Ph₂SiH₂ and PhSiH₃ led to branched end-functionalized PEs terminated with the hydrosilyl functionality (i.e. br-PE-SiPh₂H or br-PE-SiPhH₂).