In this work, we describe a new synthetic strategy in organolanthanides chemistry, allowing the one-pot preparation of two unprecedented families of heterobimetallic half-lanthanidocenes (Nd, La) and lanthanidocenes of general formula [(CpR)Ln(BH4)3]2[Mg(THF)6] and [(CpR)2Ln(BH4)(m-BH4)]2(Mg(THF)3, respectively (CpR is a cyclopentadienyl ligand). This efficient synthetic approach, that we call the “borohydride/alkyl route”, is based on the employ of the very common trisborohydride derivatives Ln(BH4)3(THF)3, and requires a minimal experimental work-up. We will present the syntheses and X Ray structures of several complexes belonging to this new family, including a very rare example of half-neodymocene bearing just one C5H5 ligand. The isolated complexes afford efficient precatalysts for selective and controlled ethylene, isoprene and styrene polymerization.