Lewis base free hydrido complexes of yttrium and lutetium supported by bulky cyclohexyl- substituted guanidinato ligands, [Ln(mu-H){(Me3Si)(2)NC(NCy)(2)}(2)](2) (Ln = Y, Lu), were synthesized and characterized. Single-crystal X-ray diffraction studies revealed dimeric structures. H-1 NMR spectroscopy shows that complex [Y(mu-H){(Me3Si)(2)NC(NCy)(2)}(2)](2) retains its dimeric structure in C6D6 solution. Scrambling of the hydrido complexes [Ln(mu-H){(Me3Si)(2)NC(NCy)(2)}(2)](2) (Ln = Y, Lu) in C6D6 resulted in an equilibrium mixture containing the heterodimetallic species [{(Me3Si)(2)NC(NCy)(2)}(2)Y(mu-H)(2)-Lu{(Me3Si)(2)NC(NCy)(2)}(2)], indicating the dissociation of dimers and the presence of monomeric species in solution. Both compounds initiate the polymerization of ethylene: the activity of the cyclohexyl- substituted yttrium complex, [Y(mu-H){(Me3Si)(2)NC(NCy)(2)}(2)](2), is much lower than that of the isopropyl-substituted analogue, [Y(mu-H){(Me3Si)(2)NC(NiPr)(2)}(2))(2), while in the case of lutetium the activities of [Ln(mu-H){(Me3Si)(2)NC(NR)(2)}(2)](2) (R = iPr, Cy) are similar. Complexes [Ln(mu-H){(Me3Si)(2)NC(NR)(2)}(2)](2) (Ln = Y, Lu; R = iPr, Cy) were shown to catalyze efficiently the hydrosilylation of 1-nonene with PhSiH3 (at a 1:1 substrates mol ratio) to give the terminal silane PhSiH2(n-C9H19) exclusively. If the hydrosilylation reaction is carried out in the presence of a twofold molar excess of 1-nonene, double addition takes place and leads to the formation of tertiary silane PhSiH(n-C9H19)2, which was obtained in 96% yield. The hydrido complexes [Ln(mu-H){(Me3Si)(2)NC(NiPr)(2)}(2)](2) (Ln = Y, Lu) efficiently initiate the ring-opening polymerization of epsilon-caprolactone to give polymers with molar mass up to 80000. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).< Réduire