The direct reactions of (C5H5)(2)LnCl with LiN=C(NMe2)(2) proceeded at room temperature in THF under pure nitrogen to yield the lanthanocene guanidinate complexes [(C5H5)(2)Ln(mu-eta(1):eta(2)-N=C(NMe2)(2))](2) (Ln = Gd (1), Er (2)). Treatment of phenyl isocyanate with complexes 1 and 2 results in monoinsertion of phenyl isocyanate into the Ln-N(mu-Gua) bond to yield the corresponding insertion products [(C5H5)(2)Ln(mu-eta(1):eta(2)-OC(N=C(Nme(2))(2))NPh)](2) (Ln = Gd (3), Er (4)), presenting the first example of unsaturated organic small molecule insertion into the metal-guanidinate ligand bond. Further investigations indicate that N,N'-diisopropylcarbodiimide does not react with complexes 1 and 2 under the same conditions; however, it readily inserts into the lithium-guanidinate ligand bond of LiN=C(NMe2)(2). As a synthon of the insertion product Li[((PrN)-Pr-i)(2)C(N=C(NMe2)(2))], its reaction with (C5H5)(2)LnCl gives the novel organolanthanide complexes containing the guanidinoacetamidinate ligand, (C5H5)(2)Ln[((PrN)-Pr-i)(2)C(N=C(NMe2)(2))] (Ln = Yb (5), Er (6), Dy (7)). All complexes were characterized by elemental analysis and spectroscopic properties. The structures of complexes 1, 3, 5 and 7 were determined through X-ray single-crystal diffraction analysis.