Unusual insertion of carbodiimide into the Y-Cp(Cp=C5H5) bond and isomerization of the resulting cyclopentadienyl (Cp)-substituted amidinate complex to the amidino-substituted Cp complex have been established, representing an efficient and simple method for ring modification of sensitive metallocenes. All products, including the rare four-center interaction precursor of the insertion, have been characterized by X-ray structural analyses.