The preparation and characterization of guanidinate-containing complexes of Nb and Ta is described. The direct reactions of M(NMe2)(5) with either dicyclohexylcarbodiimide (CyN=C=NCy) and diisopropylcarbodiimide ((PrN)-Pr-i=C=(NPr)-Pr-i) proceeded smoothly at room temperature under nitrogen to yield [RNC(NMe2)NR]M(NMe2)(4) (M = Ta, Nb; R = Cy, Pr-i). The spectroscopic characterization of these materials is consistent with a symmetrical chelating bidentate guanidinate anion bonded to a pseudo-octahedral metal center. Confirmation of these details was provided by a single-crystal X-ray diffraction study in the case of [CyNC(NMe2)NCy]Ta(NMe2)(4) (1). Delocalization of the lone pair of electrons on the guanidinate NMe2 group into the ligand N-C-N pi system does not appear to be significant in these species.