The reactions of the bis(alkyl) complexes [Ap*Ln(CH2SiMe3)2(thf)] (Ln = Y, Lu) with both PhSiH3 and H2 result in selective assembly of the novel trinuclear rare-earth alkyl hydrido clusters [(Ap*Ln)3(μ2-H)3(μ3-H)2(CH2SiMe3)(thf)2]. Both cluster compounds are single-component ethylene polymerization catalysts.