Previous results obtained for the cis-polymerisation of isoprene with scandium half-sandwich complex Cp*Sc(BH₄)₂(THF) 1a were extended to its neodymium analog. The X-ray structure of the already reported neodymium monomer compound Cp*Nd(BH₄)₂(THF)₂1b is presented. Cp*Nd(BH₄)₂(THF)₂/[CPh₃][B(C₆F₅)₄]/Al(ⁱBu)₃ catalytic system was found to be very active and stereoselective towards isoprene polymerisation, leading to highly 1,4-cis polyisoprene up to 92%. The effect on isoprene polymerisation of the addition of a NHC molecule to Cp*Ln(BH₄)₂(THF)_n pre-catalyst (Cp* = η⁵-C₅Me₅, Ln = Sc, n = 1, 1a; Nd, n = 2, 1b; Sm, n = 2, 1c) or to a trisborohydride Ln(BH₄)₃(THF)_n (Ln = Sc, n = 1.5, 2a; Nd, n = 3, 2b; Sm, n = 3, 2c) was also studied. Several NHC ligands were assessed: the classical [1-C{(N^tBuCH)}₂] (L¹) and functional N-heterocyclic carbenes, two amino-tethered HNBu^tCH₂CH₂[1-C{N(CHCH)NR}] (HL^2-R) (R = ^tBu, Mes (Mes = 2,4,6-Me₃-C₆H₂)) and the hydroxyl-tethered HOCMe₂CH₂[1-C{N(CHCH)NⁱPr}] (HL³). Neodymium-based complex (L^2-tBu)Nd(BH₄)₂(THF)₂3 could be isolated and characterized. With some of the catalytic combinations tested, the introduction of the NHC ligand in the coordination sphere of the complex induces a switch of the stereoselectivity of the resulting polymer. Scandium complex 2a, which produces mainly 1,4-cis polyisoprene when associated to a borate activator and aluminum alkyl, leads to 1,4-trans polymer up to 94% regular when HL^2-tBu carbene is added to the same reaction mixture. This result is the only example of highly trans-polyisoprene synthesized with a scandium based catalyst. Coordination of the carbene moiety to the rare earth centre is confirmed by NMR studies on paramagnetic neodymium pre-catalysts.