Although the nucleophilic alkylation of aromatics has recently been achieved with a variety of potent main group reagents, all of this reactivity is limited to a stoichiometric regime. We now report that the ytterbium(II) hydride, [(BDIYbH)-Yb-Dipp](2) (BDIDipp=CH[C(CH3)NDipp](2), Dipp = 2,6-diisopropylphenyl), reacts with ethene and propene to provide the ytterbium(II) n-alkyls, [(BDIYbR)-Yb-Dipp](2) (R=Et or Pr), both of which alkylate benzene at room temperature. Density functional theory (DFT) calculations indicate that this latter process operates through the nucleophilic (S(N)2) displacement of hydride, while the resultant regeneration of [(BDIYbH)-Yb-Dipp](2) facilitates further reaction with ethene or propene and enables the direct catalytic (anti-Markovnikov) hydroarylation of both alkenes with a benzene C-H bond. Nucleophilic alkylation of aromatics with main group reagents was achieved, but it is limited to a stoichiometric regime. Here, the authors report that the ytterbium(II) hydride reacts with ethene and propene to afford ytterbium(II) n-alkyls, both of which can facilitate the catalytic alkylation of benzene at room temperature via an S(N)2 mechanism.