The complexes [Ni(η3-CH2CHCH2)Br(κ1P-PR2CH2CHCH2)] (R = Ph 1, iPr 2) and [Ni(η3-CH2C(R′)CH2)(κ1P-PR2CH2CHCH2)2][BAr′4] (R′ = H, R = Ph 4a, R = iPr 4b; R′ = CH3, R = Ph 5a, R = iPr 5b; Ar′ = 3,5-C6H3(CF3)2) have been prepared and characterized. The X-ray crystal structures of 1, 2 and 5b have been determined. 4a–b and 5a–b are catalyst precursors for the oligomerization of RC6H4CHCH2 to oligostyrene (R = H) or oligo(4-methylstyrene) (R = CH3) respectively, without the need of a co-catalyst such as methylalumoxane. The catalytic activities range from moderate to high. The oligomerization reactions are carried out in the temperature interval 25–40 °C in 1,2-dichloroethane, using an olefin/catalyst ratio equal to 200, yielding oligostyrenes with a high isotactic fraction content Pm, with Mn in the range 700–1900 Dalton, and polydispersities between 1.22 and 1.64. The cationic complexes 4a–b and 5a–b are also effective catalyst precursors for the hydrosilylation reactions of styrene or 4-methylstyrene with PhSiH3 in 1,2-dichloroethane at 40 °C using an olefin/catalyst ratio equal to 100, leading selectively to RC6H4CH(SiH2Ph)CH3 (R = H, CH3) in 50–79% yield.